This study investigated whether organizational characteristics of mothers’ verbalizations (i.e., co-construction skills) when creating attachment-related stories with their children are related to maternal behavioral sensitivity and child behavioral attachment security. Fifty-four 2.5-year-olds and their mothers participated in three visits: lab, home, and playground. Three structural characteristics of maternal verbalizations were assessed during a joint storytelling task in the lab: “creating a co-constructive atmosphere,” “encouraging content elaboration,” and “supporting explanatory frameworks.” Observations of maternal sensitivity and child attachment security were completed both at home and playground; composites of sensitivity and security across contexts were used in analyses. As expected, sensitivity and security were significantly associated. Co-construction skills were significantly associated with maternal sensitivity. Contrary to expectations, co-construction skills were not related to child security. Theoretical and methodological implications are discussed.