This article tests the cross-national equivalence of the political protest scale, as developed by Barnes and Kaase, in 20 Western European countries using a battery of items included in the fourth wave of the European Values Study. The scale measuring the concept of political protest is widely used, but no evidence of cross-country equivalence has yet been provided in the literature. The article illustrates the concept of political protest, the relationship between concept formation, operationalization, and measurement equivalence, and the possible consequences of a lack of equivalence. It is argued that comparative research may be threatened by a lack of measurement equivalence. The spread of international surveys eases comparative designs, but at the same time enlarges the chances that we compare what is not actually comparable. The article then outlines an empirical strategy to assess the political protest scale's measurement equivalence. To assess cross-country equivalence, Mokken Scale Analysis, a nonparametric scaling method within the family of Item Response Theory models, is used. This has been shown to work better than Confirmatory Factor Analysis when dealing with dichotomous and polytomous items forming ordinal scales. The results show that the cross-country equivalence of the political protest scale depends on the type of measure the scholar wishes to build and use.