In an experimental design, we examined the effects of middle-term training in social information processing (SIP) and in Theory of Mind (ToM) on preschoolers' social cognition and social adjustment. 48 preschoolers took part in a pre-test and post-test session involving cognitive, socio-cognitive and social adjustment (direct and indirect) measures. They were allocated at random to an experimental group (subgroups of 4 children attended 15 45-minute sessions in which they received ToM and SIP training using educational materials) or a control group (free play sessions). The results showed that each group improved in direct measures of social cognition. The experimental group was significantly better at the SIP task in post-test than the control group. Significant correlations were obtained between individual and family characteristics and direct measures. Furthermore, regression analyses showed that, after controlling for pre-test result, SIP measures in post-test were significantly predicted by pre-test results and group condition. This study could help professionals and parents by providing them with some basis for early training exercises that can induce an improvement in SIP competences in preschoolers.