We have applied the technique of positron annihilation lifetime (PAL) to study the nature and the characteristics of polyisoprene network structures produced by three distinctly different crosslinking systems. The crosslinking systems are: Dicumly peroxide (DCP, which leads to carbon-carbon crosslinks), tertramethylthiuram disulfide (TMTD) accelerated sulfur crosslinking and 2-bisbenzothiazole-2,2 ' -disulfide (MBTS) accelerated sulfur crosslinking. The positron annihilation lifetime measurements give information about the fractional free volume, the mean free volume hole size and the distribution of the free volume hole size for the vulcanizates. The inhibition effect of the curatives needs to be taken into account when interpreting the various positron lifetime parameters for the crosslinked samples. Each of the different crosslinking systems produces distinctly different trends, These differences are interpreted in terms of the network structures, which are in turn related to the differences in the mechanisms of the crosslinking reaction.