Influence of water absorption on internal space charge behaviour of an aramid/epoxy composite was firstly discussed based on the experiments using specimens after water absorption pre-treatment. More hetero charges were observed in the specimen treated at higher temperature. Chemical analyses suggested that the increase of hetero charges was due to the increase of ion impurities. Since insulating materials for printed circuit boards are generally tested at high temperature and high humidity, we have developed an in-situ space charge measurement system in an environmental chamber using the pulsed electroacoustic method. Hetero charges observed in the aramid/epoxy resin increased significantly at 40°C and 90 % RH. This insitu space charge measurement should contribute to understand the ionic migration phenomena as well as to evaluate the test conditions of printed circuit boards.