Monobasic carboxylic (fatty) acids CnH2nO2 among values n = 11–21 (homologue purity 98-99 %) have been studied by the X-Ray powder diffraction method. All origin samples (chemical agent) of odd acids are two-phase. Two monoclinic modifications M1 (space group P21/a) and M3 (space group P21/с) were discovered among acids with n = 15, 17, 19 and 21, triclinic Tc (space group P-1) and bilayered monoclinic 2M (space group A2/a) – acid with n = 13, triclinic Tc (space group P-1) and monoclinic M3 (space group P21/с) modifications – acid with n = 11. All samples of even acids (n = 12, 14, 16, 18, 20, 22 and 24) are characterized by monoclinic modification M2 (space group P21/a). Exception – stearic acid (n = 18). It could crystallized in two monoclinic M2 (space group P21/a) and M4 (space group P21/a) and orthorhombic Or (P21212) modifications. Express diagnostic method of even and odd fatty acids of different modifications is developed.