A systematic study on the viscosity and refractive index of 1-alkyl-3-methylimidazolium based ionic liquids (alkyl = methyl, butyl and hexyl) combined with three phosphorus containing anions, i.e. dimethylphosphate, methyl methylphosphonate and methylphosphonate, is reported. Experimental measurements account for temperature effects, while the refractive index is determined at multiple wavelengths in the visible and near infrared region. Despite the structural similarity of these anions, significant differences in the physical properties of the resulting ILs are identified, along with the clear trend of viscosity increase - and refractive index decrease - with increasing alkyl chain length on the cation. Ab initio theoretical calculations are carried out to support and rationalise the observed behaviour.