The local free volume size nu, the glass transition temperature T-g and the specific volume have been estimated for propylenes branched with a defined amount (0 to 20 mole-%) of various alpha -olefins (C-4 to C-16). From the specific volume the fractional free volume was calculated and from this the number density of holes has been estimated. It is observed that T-g decreases and the hole size nu increases linearly with both the number and length of branches. From this a linear relation between T-g and nu follows. For polyethylene, an increasing number and size of holes with increasing degree of branching was observed.