Given the primacy of early relationships for child health, development and wellbeing, we developed an emotional synchrony (ES) scale; an observer-based assessment of dyadic interaction behaviors that translate mutual emotional engagement. Drawing on broader literature, we hypothesize that stronger ES may be a mutual experience particularly conducive to neural synchrony, thus leading to optimal child socioemotional development. Here, we investigate the behavioral correlates of dyadic ES using video-based data obtained from the COMBO Initiative relational health observational assessment. A subset of 46 videos was macro-coded with the ES scale and micro-coded using Boris to gather timed-sequential data with millisecond precision on maternal and infant interaction behaviors like gaze, touch, reaching, leaning in, back arching, etc. The subset was composed of 32.21±5.19 year old mothers and their 5.51±2.33 month old infant (n=28 males, 60%). The mean ES score was 50.48±24.92, which represents 48% (n=22) of the dyads scoring high, 24% (n=11) scoring low on emotional synchrony. We identified stronger ES to be associated with more playful/stimulating touch (r=.31, p=.04), mutual eye gaze (r=.73, p<.001), infant reaching for mother (r=.31, p=.04) and maternal vocalization (r=.36, p=.01). Higher dyadic ES at 4 months also predicted fewer child behavioral problems at 2 to 3 years of age (-.17, p<.05). The contribution and patterning of microcoded maternal and infant interaction behaviors is being further investigated using sequential patterning mining techniques to identify the fundamental sequential patterns of multimodal behaviors during moments of high and low ES.