We have studied the surface resistivity and charge decay times of different kinds of electrostatic discharge (ESD) protective materials as a function of relative humidity. The measurements have been done in the humidity range of 5–70% RH. The studied materials are frequently used in personnel and process protection and in packaging. Both surface resistivity and charge decay measurements are required to obtain reliable information on the performance of the materials in ESD control. According to the results, a great care should be done in the ESD management if conditions below 20–30% RH are anticipated. In such conditions some ESD protective materials may become insulators.