Research on maternal socialization of child emotion regulation often involves measures of general parenting, yet little research has considered how maternal emotion regulation and emotion expressivity relate to children's ability to regulate their emotions. Because emotion regulation can be viewed as intergenerational, mothers who display higher levels of positive emotions and lower levels of negative emotions may create a more optimal emotional climate for children to learn and practice emotion regulation, aiding in the intergenerational transmission of optimal emotion regulation. We tested a mediation model where maternal positive expressivity was hypothesized to mediate the relation of maternal emotion regulation to child emotion regulation. We also examined maternal negative expressivity as a moderator of the association of maternal positive expressivity to child emotion regulation. Maternal emotion regulation, measured as the use of reappraisal, and maternal expressivity were self-reported when children were 4-5 years old (T1). Child emotion regulation, measured as effortful control, was observed at T1. When children were 8-9 years old (T2), a summary score of child emotion regulation was computed from observed and teacher-reported effortful control. Higher levels of maternal reappraisal were related to more maternal positive expressivity, which in turn was associated with better child emotion regulation (T2), controlling for prior levels of child regulation (T1), only when maternal negative expressivity was low. This longitudinal moderated mediation pathway suggests that adaptive emotion regulation strategies used by mothers can be transmitted to children through maternal emotional expressions, specifically the interplay of positive and negative emotions. (PsycINFO Database Record (c) 2019 APA, all rights reserved).