Due to the success of antiretroviral (ART) medications, youth perinatally infected with HIV (PHIV+) are surviving into adolescence and young adulthood - the first generation to do so. Factors which influence non-adherence to ART in this group are of particular importance to the development of effective assessments and interventions. Within-participant research is important given the significant prevalence of inconsistent ART adherence reported in the literature. Previous studies have focused on between-participants differences and tended to ask participants to estimate their adherence over a period of time. No quantitative episodic (or event-level) investigations, related to specific incidences of adherence or non-adherence, have been conducted previously in this area. The present study aimed to address these gaps in the literature by investigating within-participant variation in episodic antiretroviral adherence informed by the Information-Motivation-Behavioural Skills (IMB) Model. The study explored situational variation between adherent and non-adherent events in information, personal motivation, social motivation, and behavioural skills. A secondary aim was to investigate whether situational differences in affect, or behavioural context were associated with episodic adherence. Twenty-nine PHIV+ young people recruited from the Adolescents and Adults Living with Perinatal HIV (AALPHI) cohort completed questionnaires measuring psychological and behavioural variables. Paired t-tests and McNemar's chi-square tests were used to analyse associations between behavioural and psychological factors related to adherent and non-adherent events. Weekend days, being away from home and a disrupted daily routine were associated with episodes of non-adherence. Lower reported behavioural skills and lower positive affect were associated with non-adherent episodes. There were no significant effects of negative affect, information, personal or social motivation. A conditional logistic regression model including behavioural skills and positive affect was significantly predictive of non-adherent episodes, although each predictor was not statistically significant independently. These findings are discussed in relation to previous antiretroviral adherence studies. Clinical and theoretical implications are discussed.