This paper contributes to the study of transitivity as a general property of the clause. Unlike most previous work on the subject, however, transitivity in the present article is used to study a lexical alternation, namely the two causative predicates dejar ‘let’ and hacer ‘make’ in Spanish. To do this, I use the transitivity index (TI), a weighted continuous measure of transitivity based on Hopper and Thompson’s (1980, transitivity in grammar and discourse, Language 56, 251–299) transitivity parameters. The advantage of the TI is that it assigns different weights to each of the transitivity parameters and it is therefore sensitive to the particular construction it is applied to. I show that the TI can correctly predict the two Spanish causatives dejar ‘let’ and hacer ‘make’ with 80% accuracy and demonstrate that hacer is associated with higher transitivity contexts. In addition, linguistic features of the causer such as grammatical person and number are found to help distinguish between the two predicates. The finding that a lexical alternation can be reduced to a difference in transitivity raises important questions regarding the structure of the lexicon and the type of information it may contain.