It is correct that generally only the business portion of travel expenses are deductible.
For example, if I go on a trip that is part business and part personal the usual method is to only deduct the days of lodging that were for the business portion of the trip. Or, the cost of a side trip for sight seeing would not be included as a business deduction.
See http://www.irs.gov/publications/p463/ch01.html#en_US_2011_publink100033773 "Trip Primarily for Business: You can deduct all of your travel expenses if your trip was entirely business related. If your trip was primarily for business and, while at your business destination, you extended your stay for a vacation, made a personal side trip, or had other personal activities, you can deduct only your business-related travel expenses. These expenses include the travel costs of getting to and from your business destination and any business-related expenses at your business destination.
Trip Primarily for Personal Reasons: If your trip was primarily for personal reasons, such as a vacation, the entire cost of the trip is a nondeductible personal expense. However, you can deduct any expenses you have while at your destination that are directly related to your business."
There are special rules when the trip is outside the United States.
"Even if you did not spend your entire time on business activities, your trip is considered entirely for business if you meet at least one of the following four exceptions.
Exception 1 - No substantial control. Your trip is considered entirely for business if you did not have substantial control over arranging the trip. The fact that you control the timing of your trip does not, by itself, mean that you have substantial control over arranging your trip.
You do not have substantial control over your trip if you:
Are an employee who was reimbursed or paid a travel expense allowance,
Are not related to your employer, and
Are not a managing executive.
"Related to your employer" is defined later in chapter 6 under .
A "managing executive" is an employee who has the authority and responsibility, without being subject to the veto of another, to decide on the need for the business travel.
A self-employed person generally has substantial control over arranging business trips.
Exception 2 - Outside United States no more than a week. Your trip is considered entirely for business if you were outside the United States for a week or less, combining business and nonbusiness activities. One week means 7 consecutive days. In counting the days, do not count the day you leave the United States, but do count the day you return to the United States.
Exception 3 - Less than 25% of time on personal activities. Your trip is considered entirely for business if:
You were outside the United States for more than a week, and
You spent less than 25% of the total time you were outside the United States on nonbusiness activities.
For this purpose, count both the day your trip began and the day it ended.
Exception 4 - Vacation not a major consideration. Your trip is considered entirely for business if you can establish that a personal vacation was not a major consideration, even if you have substantial control over arranging the trip. "
So, there must be a determination that the trip was primarily for business to deduct travel and not be limited to expenses you have while at your destination that are directly related to your business.
Even for the trip that was primarily for business it may be required to allocate your travel time on a day-to-day basis between business days and nonbusiness days.
You do not have to allocate your travel expenses if you meet one of the four exceptions listed earlier.
For more information see the Travel allocation rules at the link above about rules for travel outside the United States.
In summary, if the trip is primarily for business (including the exceptions listed as considered primarily for business) all of the travel and lodging expenses can be deducted and personal expenses are not deducted, such as a for a sightseeing side trip.
Of course, records to document the expenses and the business activity should be kept.
Please ask if you need clarification.