Separation and single-motherhood are both associated with depression, but it is unclear whether separating mothers’ depression mainly emerges during separation or afterwards, reflecting the strains of single-parenting rather than the strains of separation. Grandparents are often the most important source of support to families with children, and their potential support may moderate separating mothers’ depression trajectories. Using longitudinal Finnish register-data on 151,703 separating mothers whose youngest child is aged 12 or less, we examine their depression trajectories five years before and five years after separation. Preliminary results show that separation is associated with depression among all mothers, but the depression trajectories vary by living arrangement, child age and grandparental characteristics. Although non-custodial mothers have the highest risk of depression, custodial mothers and mothers living with younger children seem most vulnerable to prolonged post-separation depression. Grandparental support may have a protective effect against depression both before and after separation.