Mueller matrices for micrometer-sized objects (2 </= kalpha </= 15) have been calculated for spherical and randomly oriented nonspherical and inhomogeneous bodies with relative refractive indices <1.3. These matrices were found to be highly different from the Rayleigh-Debye matrix as well as from the matrices observed for particles with higher contrast. Some general conclusions have been drawn for homogeneous spherical and nonspherical particles. Layered spheres and spheres with a small imaginary part of the refractive index show some unusual polarization behavior. It was not possible to deduce general features for layered spheres because of their complexity. We found almost invariant Mueller matrices for particles with small absorption in the investigated Mie region.