This study entails the experimental characterization of the Poisson's ratio of hollow particle (microballoon) filled composites, known as syntactic foams. Syntactic foams are lightweight materials used in marine and aerospace structural applications. Such applications require low density and high damage-tolerance, which is achieved in these materials through the presence of hollow particles. Analyses on Poisson's ratio's dependence on syntactic foam composition and microstructure are presently not available. The present work is focused on measuring Poisson's ratio of eight compositions of vinyl ester-glass microballoon syntactic foams and characterizing the effect of volume fraction and microballoon wall thickness on the composites' Poisson's ratio. The specimens are tested under tensile loading conditions, and strain gage rosettes are used to measure strain in the longitudinal and transverse direction. Experimental results are obtained in order to observe the trend in Poisson's ratio values over the entire set of syntactic foams, which correlates with the change in microballoon volume fraction and wall thickness. The experimental data are expected to aid the validation of micromechanics models for syntactic foams.