The electron spin resonance (e.s.r.) spin-probe method has been applied to investigate the matrix heterogeneity of natural rubbers differing in molecular weight, molecular weight distribution and gel content. The composite temperature-dependent e.s.r. spectra above the glass transition temperature (T-g) were evaluated in order to obtain the population of spin probes contributing to restricted and fast motion. From the ratio of spin probes distributed in the two motionally different environments, microstructural heterogeneities of the rubber matrix are observed. It is shown that the restricted spin-probe motion above T-g is predominantly determined by the amount of gel phase and structure of the gel. Chemical treatment and mastication of natural rubber, which have a complex influence on gel fraction, molecular weight and intermolecular interactions, have been considered in analysis of the temperature dependence of the e.s.r. spectra. A comparison between natural rubber and a synthetic polyisoprene confirms the contribution of the gel phase and its structure to the microstructural characteristics of rubber matrices.