It is well known that proper water management inside a polymer electrolyte membrane (PEM) fuel cell is essential for obtaining high performance. Insufficient water lowers the conductivity of the membrane and yields low currents at a fixed voltage whereas excess water leads to flooding of the electrode and low currents at a fixed voltage due to a decreased reaction area. Typically the internal hydration is adjusted by the inlet humidity of the feed streams however the optimum depends on the given membrane and electrode assembly. As a first step in predicting this optimum a priori, data are presented for a 10-cm2 test cell with serpentine flow channels. Currents were measured for various unsaturated inlet humidity conditions, at different cell voltages, and for various operating cell temperatures. Data on the closure of water balances are presented and used to quantify the humidity effects on the current. These experimental data provide a basis for the numerical simulations presented in part II of this paper.