Preschool education is a privileged setting where social development and the progressive achievement of emotional adjustment occur. During this stage, affect plays an important role in social interaction. Furthermore, children's play has been described as an activity with a great potential for promoting development. This article reports results from the systematic observation of 38 five- to six-year-old children in four different school activities on two separate occasions. A total number of 304 registries, totalling 1,520 minutes, were made during the observations, which were conducted with the objective of comparing affective expression in different activities and inquiring into its relationship to school adjustment and performance in preschoolers. It was found that affect's intensity and quality is higher during play. Results are discussed taking into account positive relationships between affect and school adjustment and performance, concluding that children's play is a privileged activity for affective development and should be promoted in preschool education. (author abstract)