Mueller matrix imaging polarimetry represents a novel means of characterizing the polarization effects of optoelectronic devices. The Mueller matrix contains the complete polarization properties of a sample, and can therefore be used to calculate properties such as phase retardance, polarization dependant losses and polarization crosstalk. The complete polarization properties of a series of GaAs/AlGaAs self-imaging waveguide beamsplitters were measured with an imaging Mueller matrix polarimeter. Polarization properties were mapped across high resolution images of the devices' outcoupling faces, and the uniformity of the polarization properties was measured. Properties investigated include magnitude and orientation of linear retardance, polarization dependant losses, and crosstalk between TE and TM modes.