For a quantitative description of rubber crosslinking it is desirable to determine the crosslink density of the network. Styrene-butadiene-styrene blockcopolymers can be regraded as model networks for a rubber with uniform chainlengths between the crosslinks. For these model networks the relation between the proton spain-spin relaxation time and the crosslink density is determined. Below a critical value for the molecular weight of the rubber segment (M c = 40 000) which corresponds to the formation of entanglements, the relaxation time is proportional to the crosslink spacing. Above the critical value the relaxation time is almost independent of the moleclar weight of the rebber.