The effects of heat transfer mechanisms on the charging process in metal hydride reactors are studied under various charging pressures. Three different cylindrical reactors with the same base dimensions are designed and manufactured. The first one is a closed cylinder cooled with natural convection, the fins are manufactured around the second reactor and the third reactor is cooled with water circulating around the reactor. The temperatures of the reactor at several locations are measured during charging with a range of pressure of 110 bar. The third reactor shows the lowest temperature increase with the fastest charging time under all charging pressures investigated. The effective heat transfer coefficients of the reactors are also calculated according to the experimental results and they are found to be 5.5 +/- 1 W m(-2) K(-1), 35 +/- 2 W m(-2) K(-1) and 113 +/- 1 W m(-2) K(-1), respectively. The experimental results showed that the charging of hydride reactors is mainly heat transfer dependent and the reactor with better cooling exhibits the fastest charging characteristics. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.