The refractive index (RI) and Fourier transform infrared (FTIR) spectrum of a poly(vinyl chloride) (PVC)/polystyrene (PS) mixture were measured. The variation of RI with the mixture composition was found to be nonlinear, which indicated the immiscibility of the PVC/PS mixture. This result agrees with results obtained by the FTIR technique, which indicated negligible intermolecular interactions between the PVC/PS components, and the miscibility of the polymer mixture was limited for the compositions studied. The results obtained by RI and FTIR were compared with the results obtained by the viscosity method with criteria based on interaction parameters proposed by Krigbaum and Wall, Chee, Zhu, and Garcia et al. and the thermodynamic parameter proposed by Jiang and Han. A criterion based on a simple cluster theory proposed by Pan et al. was also applied to the PVC/PS mixture solutions to study the polymer-polymer interactions. The conclusions about PVC/PS immiscibility made by Pan et al. and Jiang and Han agreed with our conclusions obtained by FTIR and RI measurements. On the other hand, the results obtained by Chee, Zhu, and Krigbaum and Wall showed partial miscibility of the PVC/PS mixture, whereas the polymer mixture was miscible according to the Garcia et al. criterion. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2770-2777, 2011