To clarify the controversy about whether the adaptive comfort model is applicable in buildings with mechanical cooling systems operating in natural ventilation (NV) mode, we designed a longitudinal study in a mixed-mode (MM) building where mechanical air conditioning and natural passive cooling coexist in the same case building. Through statistical analysis of occupants' thermal responses during air conditioning (AC) and NV periods, considerable differences in occupants' perceived comfort were observed. The results show that occupants' actual thermal sensation and acceptance of thermal conditions both varied when the building changed from AC to NV mode, or vice versa. Compared to the steady-state comfort model (i.e., PMV), the adaptive model was found to be more applicable to MM buildings, especially when NV was being utilized. These findings provide support for a more flexible applicability of the adaptive comfort model in the real world, and can serve as a valuable reference for the design and operation of MM buildings.