While point-of-use (POU) water treatment may be efficacious in laboratory or idealized intervention settings, its use as a strategy for delivering safer drinking-water is constrained by (1) relatively poor field performance compared with laboratory performance, (2) low adherence (correct, consistent, and sustained use), and 3) insufficient understanding of underlying behavioural barriers and drivers to adherence. A multi-site, mixed-methods longitudinal crossover trial was conducted, assessing two flocculent-disinfectant POU products: the Purifier of Water® (“PoW”) and the new Pureit® sachet, in urban Zambia and peri-urban Pakistan. The aim was to evaluate field performance, different measures of adherence (chlorine residuals, used packet counts, and self-reported usage), and potential correlates of adherence over time. (1) Pureit-treated samples had significantly higher chlorine residuals in both countries, though did not maintain minimum levels of free chlorine any longer, and had potentially weaker buffering capabilities than PoW. Field performance also varied significantly between study sites, and was sensitive to differences in adherence, measurements, and reporting accuracy. Qualitative feedback indicated a number of product-related weaknesses. (2) Adherence was generally low and declined over time in both countries, while untreated water consumption rose. Adherence was higher in Pakistan than in Zambia. Self-reported usage was considerably higher than observed measures. (3) A complex interplay was observed between drivers and barriers. The perceived need to treat water may have ultimately been purposive, based on circumstantial factors that influenced quality perceptions. Both products’ adherence-related costs ultimately outweighed their benefits. This study’s findings underline the challenge in high POU adherence, the importance of carefully measuring field performance, and suggest that for POU to be consistently used – and thus deliver health impact – behavioural factors and added-value to the user should inform intervention and design strategies alongside effectiveness, even under short-term use conditions.