A new representation of the viscosity of isobutane has been developed. The representative equations include zero-density and initial-density dependence correlations. The higher density contributions to the residual viscosity are formed by a combination of double polynomials in density and reciprocal temperature and of a free-volume term with a temperature-dependent close-packed density. The new full surface correlation is based on primary experimental data sets selected as a result of a critical assessment of the available information. The validity of the representation extends from the triple point to 600 K and 35 MPa in accordance with the modified Benedict–Webb–Rubin equation of state by Younglove and Ely (1987). The uncertainty of the representation varies from ±0.4% in the dilute gas phase between room temperature and 600 K to ±3% in the thermodynamic ranges in which the equation of state is valid as well as where primary experimental data are available.