Many people switch sources of drinking water and sanitation between seasons, yet such shifts are not reflected in the reporting of access to improved water and sanitation services by the Joint Monitoring Programme (JMP). Drawing on quantitative and qualitative data collected from urban and rural sites in dry and rainy seasons in Benue state Nigeria, this study highlights the importance of seasonal variations in water access and quality. Water testing showed that water sources had higher levels of contamination with coliforms, nitrate and chloride in the dry season than the wet season. The contamination of water from these pollutants are above WHO standards and believed to come mainly from pit latrines. Semi-structured interviews revealed that many people who use improved water and sanitation facilities in the rainy season sometimes switch to poorer quality unimproved sources in the dry season. Travel times for collecting water as well as waiting times also significantly increased in the dry season. It is recommended that this important seasonality element is factored into JMP data collection and reporting.