This paper uses marriage registration statistics to carry out a long-term descriptive analysis of age homogamy patterns in Spain, from 1922 to 2006. We first examine the relationships between marital age homogamy patterns, trends in second order marriages and age at marriage of men and women. Second, we apply a standardization method to decompose the changes of age homogamy among first marriages into the effects produced by changes in age-sex composition and those produced by the underlying age preferences of spouses. Results show that age homogamy increased during the twentieth century, particularly in the last 30 years, while traditional marriages, in which the husband is older than the wife, are becoming less common. Although the analysis proves that under dramatic historical circumstances, changes in the age-sex composition of eligible partners modified age-assortative mating patterns, major responsibility for such patterns is to be found in behavioural factors.