Intrinsic viscosity [êta] characterizes flow, polymerization, or degradation of dilute polymer solutions. Unlike other high polymers, natural rubber hardly completely dissolves in most organic solvents, and eliminating the insoluble phase undermines the quality of the measurement and its interpretation. On using some natural and synthetic polyisoprene samples, the Huggins and Schulz-Blaschke best-fits were most suitable for estimating [êta]for very dilute polyisoprene solutions (< 0.06 g/dL in cyclohexane at 30°C). Logarithmic linearization of the Mark-Houwink-Sakurada equation ([êta] = K.Ma; M is molar mass) and a curvilinear model on untransformed data gave different Mark-Houwink constants (K and a) for polyisoprene standards, differences that seemed to have no bearing on their viscosity-average molar masses ( ). Plots of against weight-average molar mass enabled differentiation of the polyisoprenes into larger groupings, differences that could be attributed to different extents of branching or of aggregation between macromolecules.