This study aimed to gain a better understanding of the normative declines in adolescent disclosure and maternal knowledge over the course of adolescence, by assessing the underlying monitoring processes. Multilevel structural equation models were applied to 15 assessments among 479 families across five years (13 years at T1, 57% boys, 11% low SES). Developmental declines in mother-perceived disclosure and knowledge were observed, which were partially explained by processes operating at the level of the family unit. On average, mothers were more knowledgeable in weeks with more disclosure and more solicitation, and adolescent disclosure was higher in weeks with more maternal solicitation and less control. The effect sizes and even the directions of these within-family correlations varied between families, however. This heterogeneity was partially explained by the level of maternal control and adolescent disclosure, and by the families’ socio-economic status. Within-family fluctuations in knowledge and disclosure were also correlated with fluctuations in relationship quality and adolescent and mother mood. Overall, these within-family processes explained up to 14% of the normative developmental decline in disclosure and 19% of the decline in knowledge. This study thus suggests that a wide variety in monitoring processes may drive normative declines in adolescent disclosure and maternal knowledge.