This paper is a contribution to the empirical literature on R&D cooperation. It explores the variables that determine a firm's R&D collaborative expenditure by means of a sample of Italian firms. A tobit model, adjusted for heteroscedasticity and non-normality (Inverse Hyperbolic Sin transformation to the dependent variable), is used to deal with the large number of zero responses. Size, public grants and innovation are found to be effective in determining the level of cooperative R&D expenditure. Absorptive capacity, expressed by the in-house stable R&D effort, also plays an important role. This is in line with the idea that internal R&D is required if a firm is to take advantage of the outcomes of external R&D investment.