In this paper, the Poisson's ratio v of low-temperature plasma-enhanced chemical vapor deposited silicon nitride (SiNxHy) thin films has been determined by a modified double-membrane bulge test. This test method utilizes a square membrane and a large-aspect- ratio rectangular membrane that is fabricated alongside from the same thin film. The Poisson's ratio is determined from the ratio of the bulge deflections of the two membranes under an applied pressure. The method is suitable for determining v of either stress-free thin films or those containing low tensile residual stresses. Poisson's ratio values of 0.23 +/- 0.02 and 0.25 +/- 0.01 were measured for SiNxHy films that were deposited at 125 degrees C and 205 degrees C, respectively.