This article documents differences in cognitive development, as measured by the Peabody Picture Vocabulary Test (PPVT), between children from households with high and low socioeconomic status (SES) in two different phases of early childhood in four developing countries. A large number of potential mediators, such as urban residence, preschool attendance, early nutrition, caregiver's education, and primary school attendance are discussed. Overall, the SES gradient is reduced but persists in most countries even when controlling for all the mediators. The mediational analysis shows that, while urban residence, caregiver's education and early nutrition appear as significant mediators of the SES-PPVT relation for all countries and most ages, the size of the effect varies widely. For instance, after adding all mediators, the magnitude of the SES-PPVT relation drops by almost half in Peru (mainly due to urban residence), India (mainly due to caregiver's education at age 5 and urban residence at age 8) and Vietnam at age 5 (mainly due to caregiver's education). However, it only drops by one third in Ethiopia (mainly due to caregiver's education at age 5 and urban residence at age 8). The relative importance of each mediator also changes depending on children's age. Preschool attendance only appears as a minor mediator in Ethiopia and Vietnam at age 5, while primary school attendance does not appear as a significant mediator in any country. (author abstract)