The synthesis and characterization of some pyrazoline compounds of 1,3-diketones with hydrazine derivatives, namely, 1-(S-benzyldithiocarbazate)-3-methyl-5-phenyl-5-hydroxypyrazoline (1); 1-(2-thiophenecarboxylic)-3-methyl-5-phenyl-5-hydroxypyrazoline (2); 1-(2-thiophenecarboxylic)-3,5-dimethyl-5-hydroxypyrazoline (3); 1-(S-benzyldithiocarbazato)-3-methyl-5-phenylpyrazole (4); 1-(2-thiophenecarboxylic)-3-methyl-5-phenylpyrazole (5) and 1-(S-benzyldithiocarbazate)-3,5-dimethylpyrazole (6) are reported. Studies by IR, (1H, 13C)-NMR spectroscopies and single crystal X-ray diffraction revealed that compounds (1), (2) and (3) are formed as pyrazoline, whereas (4) and (5) are formed as pyrazole derivatives only under acidic conditions. Compound (1) crystallizes in orthorhombic P212121, a = 6.38960(10) A, b = 12.9176(3) A, c = 21.2552(5) A, (2) crystallizes in monoclinic, P21/n, a = 11.3617(2) A, b = 8.4988(2) A, c = 92.8900(10) A and β = 92.8900(5)°, (3) crystallizes in monoclinic, C2/c, a = 15.9500(5) A, b = 9.3766(3) A, c = 16.6910(5) A and β = 113.825(2)°, (4) crystallizes in monoclinic, P21/c, a = 15.228(4) A, b = 5.5714(13) A, c = 19.956(5) A and β = 91.575(7)° and (6) crystallizes in orthorhombic, P212121, a = 5.3920(2) A, b = 11.2074(5) A, c = 21.885(1) A. The (3) derivative represents the first pyrazoline compound prepared from 2,4-pentanedione and characterized crystallographically. The synthesis and characterization of some pyrazoline compounds of 1,3-diketones with hydrazine derivatives, namely, 1-(S-benzyldithiocarbazate)-3-methyl-5-phenyl-5-hydroxypyrazoline (1); 1-(2-thiophenecarboxylic)-3-methyl-5-phenyl-5-hydroxypyrazoline (2); 1-(2-thiophenecarboxylic)-3,5-dimethyl-5-hydroxypyrazoline (3); 1-(S-benzyldithiocarbazato)-3-methyl-5-phenylpyrazole (4); 1-(2-thiophenecarboxylic)-3-methyl-5-phenylpyrazole (5) and 1-(S-benzyldithiocarbazate)-3,5-dimethylpyrazole (6) are reported. Studies by IR, (1H, 13C)-NMR spectroscopies and single crystal X-ray diffraction revealed that compounds (1), (2) and (3) are formed as pyrazoline, whereas (4) and (5) are formed as pyrazole derivatives only under acidic conditions. Compound (1) crystallizes in orthorhombic P212121, a = 6.38960(10) A, b = 12.9176(3) A, c = 21.2552(5) A, (2) crystallizes in monoclinic, P21/n, a = 11.3617(2) A, b = 8.4988(2) A, c = 92.8900(10) A and β = 92.8900(5)°, (3) crystallizes in monoclinic, C2/c, a = 15.9500(5) A, b = 9.3766(3) A, c = 16.6910(5) A and β = 113.825(2)°, (4) crystallizes in monoclinic, P21/c, a = 15.228(4) A, b = 5.5714(13) A, c = 19.956(5) A and β = 91.575(7)° and (6) crystallizes in orthorhombic, P212121, a = 5.3920(2) A, b = 11.2074(5) A, c = 21.885(1) A. The (3) derivative represents the first pyrazoline compound prepared from 2,4-pentanedione and characterized crystallographically.