This paper investigates variation in the design of labor provisions in preferential trade agreements (PTAs) by focusing on the power of trade unions, the role of government partisanship, and the relative strength of skilled labor. We expect strong trade unions and left-leaning governments to be associated with more, and more far-reaching labor provisions in PTAs. We also expect the strength of skilled workers relative to the strength of unskilled workers to negatively correlate with the depth of labor provisions in PTAs. In addition, the effect of trade unions should be conditional on both the presence of left government and democracy. We test these hypotheses relying on an original dataset of labor provisions included in 483 PTAs signed between 1990 and 2016. This dataset covers 140 different labor provisions that relate to six overarching dimensions. The quantitative analysis finds support for the expectations concerning the influence of trade unions and the role of a country’s skill profile.