We analyze the occupational mobility of immigrants between their origin countries and Spain and its determinants. We use microdata from the Encuesta Nacional de Inmigrantes to compute an internationally harmonized occupational status index (ISEI) that permits to quantify and properly analyze this kind of mobility. The obtained evidence shows that, in general, immigrants experience a strong occupational downgrading in Spain when compared to their origin countries. This fact is due to the strong downgrading they experience when entering the Spanish labour market and their low improvement in the following years.