Temperature dependence of the shear viscosity measured for isotropic liquids belonging to the three homologous series: 4-(trans-4'-n-alkylcyclohexyl) isothiocyanatobenzenes (CnH2n+1 CyHx Ph NCS; nCHBT, n = 0 divided by 12), n-alkylcyanobiphenyls (CnH2n+1 Ph Ph CN; nCB, n = 2 divided by 12) and 1,n-alkanediols (HO(CH2)(n)OH; 1,nAD, n = 2 divided by 10) were analysed with the use of Arrhenius equation and its two modifications: Vogel-Fulcher and proposed in this paper. The extrapolation of the isothermal viscosity of 1, n-alkanediols (n = 2 divided by 10) to n = 1 leads to an interesting conclusion concerning the expected viscosity of methanediol, HOCH2OH, the compound strongly unstable in a pure state.