The liquefaction response of partially saturated loose sands was experimentally investigated to assess the effect of partial saturation on the generation of excess pore water pressures. An experimental setup including a cyclic simple shear liquefaction box was devised and manufactured. The box includes pore pressure and displacement transducers as well as bender elements and bending disks to monitor the response of partially saturated specimens. Uniform partially saturated specimens with controlled density and degree of saturation were prepared by wet pluviation of powdered sodium perborate monohydrate mixed with Ottawa sand. The reaction of the sodium perborate with pore water released minute oxygen bubbles, thus reducing the degree of saturation of the specimens. The uniformity of a specimen was confirmed with S wave velocity measurements and a high-resolution digital camera. The P wave velocity measurements could only confirm the presence of partial saturation but not the degree of saturation. Partially saturated specimens with varying relative densities and degrees of saturation when tested under a range of cyclic shear strains do not achieve initial liquefaction defined by maximum pore pressure ratio (r(u, max)) being 1.0. For a given degree of saturation and cyclic shear strain amplitude, the larger the relative density, the smaller is r(u, max). For a given degree of saturation and relative density, the larger the shear strain amplitude, the larger is r(u, max). The excess pore pressure ratio (r(u)) can be significantly smaller than r(u, max) depending on the number of cycles of shear strain. Tests on the sustainability of partial saturation under upward flow gradient and base excitation led to the conclusion that the specimens remained partially saturated without significant change in the degree of saturation. Based on the experimental test results presented in this paper, an empirical model for the prediction of r(u) in partially saturated sands under earthquake excitation is presented in a companion paper. (C) 2013 American Society of Civil Engineers.