Individual ethnic boundary crossing, i.e. the change in the ethnic category of an individual, is one of the elementary strategies of ethnic boundary making. The large bulk of the empirical literature so far has shown that ethnic categorization is correlated with cultural and economic attributes of the individuals. However, few empirical studies are able to investigate individual boundary crossing. This article adds to this literature by studying ethnic boundary crossing between 2001 and 2010 with a panel of about 340,000 Ecuadorians. It is shown that while ethnic boundary crossing is common, the overwhelming majority of crossings were made from or to mixed categories. In order to explain these boundary crossings, I test a cultural assimilation and an economic “money whitens” hypothesis. I find evidence for both explanations with cultural assimilation having a much stronger effect on ethnic categorization.