The viscoelastic properties of various crosslinked natural rubbers, NR, were investigated by mechanical spectroscopy. The glass transition temperature, T-g, was found to be dependent on both the crosslink density and the crosslink type. Higher values of T-g were obtained for sulfur-crosslinked NR than for peroxide-crosslinked NR at the same crosslink density. The greater influence of the sulfur content on T-g may be attributed to polysulfidic crosslinks and cyclic sulfide structures favored at high sulfur contents. Sulfur-vulcanized NRs with monosulfidic crosslinks, favored at relatively high accelerator/sulfur ratios, have properties more similar to the peroxide-cured NR with simple carbon-carbon crosslinks covalent bonds, resulting in only small shifts in T-g. A qualitative analysis of monosulfidic crosslinks and polysulfidic structures was performed with C-13 solid-state NMR spectroscopy. The storage modulus, E’, in the rubbery plateau region increased with increasing crosslink density. However, the crosslink type did not influence the moduli values as much as it influenced the T-g values. Different methods of detecting the crosslink density were also discussed.