I was recently asked what the difference between lǚ yóu 旅游 and lǚ xíng 旅行 is, both of which can be translated as “travel”.

The fact is that if you’re asking these kinds of questions your approach to language learning is all wrong. Knowing the very specific, very slight semantic differences between two words of almost identical meaning is not useful, and does not further your ability to use these words correctly in the right contexts.
It’s much more useful to find out what the most common collocations are.

Basically, what other words do these words most naturally combine with and are there any set phrases where the two can’t be used interchangeably? (gù dìng dā pèi 固定搭配)

The exact difference is not useful for the student of Mandarin and even Chinese people don’t know what the difference is (if there is any difference at all).
That said, once the question was asked I stubbornly wanted to find an answer, so I asked the question on zhī hū 知乎 (the Chinese equivalent of Quora or Reddit).

Below is the most upvoted answer. This answer is of course all in Chinese. If your Chinese level is not high enough to read this answer then it is certainly not high enough for you to need to know the exact semantic differences between these two words.

 

「旅游」与「旅行」有什么区别？

上面很多人的解释太过诗意，却离真相比较远。所有离开居住地的行为，都可以称之为旅行。比如你出差去上海开个会，或者去纳斯达克敲个钟，都是旅行。只有围绕景点展开的活动，才叫旅游。周末开车去百花山，或者到塞班岛度个假，都是旅游。

国家旅游局不能叫国家旅行局，因为旅行实在太宽泛了。

另，非要把去丽江住一周，或者到三亚游个泳也说成“旅行”的，不在讨论范围内。