This study investigated predictors of attachment security in a play context using a sample of 71 mothers and their 6-month-old infants. We sought to rethink the concept of maternal sensitivity by focusing on mothers' ability accurately to read the mental states governing infant behaviour. Five categories were devised to assess this ability, four of which were dependent on maternal responses to infant behaviours, such as object-directed activity. The fifth, mothers' Appropriate mind-related comments, assessed individual differences in mothers' proclivity to comment appropriately on their infants' mental states and processes. Higher scores in this fifth category related to a secure attachment relationship at 12 months. Maternal sensitivity and Appropriate mind-related comments were independent predictors of attachment security at 12 months, respectively accounting for 6·5% and 12·7% of its variance. We suggest that these findings are in line with current theorising on internal working models of attachment, and may help to explain security-related differences in mentalising abilities.