This longitudinal and naturalistic study aims to describe and compare grandparent and infant emotional expressions that precede, accompany and follow spontaneous imitation in the course of their dyadic interaction. Sixteen Greek, Cretan infants were video-recorded in the course of spontaneous dyadic interactions with grandfathers and grandmothers at home from the 2nd to the 10th month of their life. Microanalyses of grandmothers’, grandfathers’ and infant grandchildren’s emotional expressions within well-defined units of interaction provided the following evidence: (a) Grandparents increased pleasure-interest expression while single pleasure and interest decreased in the course of imitation. Grandparent neutral expression remained stable, at low levels, before, during and after imitation. Grandfathers were more interested than grandmothers and grandmothers expressed more pleasure-interest towards their infant grandchildren, compared to grandfathers; (b) Similar patterns of infant grandchildren’s emotional expressions in interactions with grandfathers and grandmothers provided evidence that infant interest predominated over the remaining expressions before imitation, it decreased in the course of it and it increased after it. Infant pleasure remained stable before and in the course of imitation and decreased slightly after it. Infant pleasure-interest expression increased and neutral expression decreased in the course of imitation. Infants were more neutral in interactions with their grandfathers, compared to grandmothers, particularly in the course of the first age level (2 to 3.5 months); (c) Infant age correlated with certain infants’ and grandparents’ emotional expressions. This evidence reinforces the view that grandparents are communicative partners to their infant grandchildren.