While flexible working-time arrangements are highly sought-after entitlements for the reconciliation of work and family responsibilities, they also risk workplace stigma and deterioration in users’ (typically mothers’) employment status and career prospects. Among potential influences on these outcomes is the formality of provisions. In this analysis we examine job satisfaction, work/family benefits and employment penalties among Australian mothers using flexible working arrangements, comparing the outcomes of those who accessed flexible working-time arrangements formally (through policy provisions available at their workplace) with the outcomes of those who used informally negotiated arrangements. Using data from a nationally representative cohort of working mothers, we show that the use of flexible working-time arrangements was positively associated with specific indicators of job satisfaction, and that this association was stronger among mothers who accessed formal arrangements. However, formality was positively associated not only with perceived benefits (such as more time with family) but also with problems (such as intensified workloads and lost opportunities for training). Overall, our findings suggest that while formality is important, it may also amplify the tension between benefits and penalties associated with flexible working-time arrangements.