The self-association of cis-9-octadecen-1-ol in the pure liquid state and in decane solutions has been studied mainly through near-infrared spectroscopic observation over the temperature range 298.1-348.1 K. The analysis of the sharp band at 1410 nm (the first overtone band of OH-stretching vibration mode attributed to free OH-monomer and partly to OH-polymer) leads to a conclusion that the apparent degree of association, n, of the polymer of cis-9-octadecen-1-ol is 2.37-3.78 (average 2.95) both in the pure liquid state and in decane solutions; the n value has a tendency to decrease with an increase in temperature. The apparent association constant, K, which is relatively low, also decreases with an increase in temperature. The low K value, which means small portions of polymerized molecules, is most Likely responsible for the high selfdiffusion and the low viscosity of cis-9-octadecen-1-ol, compared with those of cis-9-octadecenoic acid, existing almost entirely as cyclic dimers.