A simple spring model for molecular vibrations is applied to non-linear triatomic ABC-type molecules with Cspoint group. The development is in terms of Cartesian co-ordinates, which offers certain advantages for extensions to lattice dynamics calculations. Analytical expressions for the three normal mode frequencies are derived in terms of two stretching and one bending force constants. The model is applied to a number of ABC-type molecules and to asymmetric isotopic species of AB2-type non-linear molecules. The validity of the model is demonstrated by the satisfactory agreement between calculated and observed frequencies of isotopic species. Examination of the calculated eigenvectors shows that mixing between the A-B and B-C stretches and ABC bend is appreciable. For DOC1 a new value of the bending frequency is suggested.