The dependence of P=S vibrational frequencies, nP=S, on the substituents for some organo-P compounds was studied. The effect of the substituents on nP=S probably is the inductive and the vibrational coupling effect. The ave. frequency of nP=S could be expressed as equation: nP=S = 423 + 29 SX + 20 (M - N). The 2nd term in the right side of the equation is determine by the inductive effect and the 3rd term reflects the vibrational coupling effect. SX Is the sum of the Pauling electronegativity of the substituents, M and N are resp. the no. of P-X (X = S, Cl, or Br) and P-Y (Y = C, N, O, or F) which are the single bond of the substituents connected directly to P atom.

Key words: ORGANO PHOSPHORUS COMPOUNDS, FREQUENCY OF VIBRATION, SUBSTITUENT GROUP, INDUCTIVE EFFECT