Transparent single crystals of different phenols are grown from the melt or from saturated solution in various solvents. Crystal structure determination are studied by X-Ray or neutron diffraction methods. Among them several are used to measure physical properties. For this paper we choose non linear optical properties. Measurements first made on powder to know the importance of the phenomena have to be repeated on single crystals in several directions to know coefficients of the NLO tensor. At present time data are known for 3-methyl-4-isopropylphenol (tetragonal - space group P41). These negative uniaxial crystals are found to have NLO coefficients (d333, d331) = (1.1, 1.4) x 10-9 esu respectively. On the other hand, data show that nx at 0.53 μ is almost equal to nz at 1.06 μ (Δn = 0.0023). This fact means that the coherence length for the second harmonic generation at 1.06 μ is very large. If in fact the two indices were identically equal, we would have a very useful crystal as a device for doubling the readily available 1.06 μ source up to 0.53 μ, particularly since there are only few crystals known which have this quality. Same measurements have to be made on an isomorphous compound, 4-isopropylphenol. The two compounds syncrystallizes to give solid solution ; so it will be interesting to grow, if possible, single crystals of intermediate composition and made NLO measurements on them.