Primary objective: Research has provided evidence of the intersubjective function of imitation in grandparent-infant interaction based on the basic aspects of imitation. This lacks the systematic investigation of behaviour dynamics framing spontaneous imitation. The aim of this study was to compare the dyadic expressive behaviours (vocal, kinetic and vocal-kinetic combinations) preceding and following spontaneous imitative exchanges in grandmother-infant (GM-I) and grandfather-infant (GF-I) interactions. Method: Sixteen infants were video-recorded during spontaneous dyadic interactions with maternal grandmothers and grandfathers at home from the second to the tenth month of life. Results: Before and after imitation, there was a similarity between the GM-I and GF-I interactions, in that: (a) the communicative expressive behaviours predominated over non-communicative behaviours, (b) grandparent vocal/kinetic behaviours predominated over vocal and kinetic behaviours and (c) infant kinetic behaviours predominated over vocal and vocal/kinetic behaviours. Conclusion: It is assumed that imitation serves an intersubjective and transitional function in early grandparent-infant interaction.