Modulation of polarization is commonly employed in optical interferometry through the useof polarizers and quarter-wave retarders. Phase shifts between interfering beams can beeasily controlled with such techniques. This communication describes some details ofmodulation of polarization which are useful in phase shifting interferometry appliedto the study of phase objects. As an application, the case of a two-beam phasegrating interferometer is discussed on the grounds of polarization analysis as anexample. The configuration presented does not require micro-polarizer arrays oradditional software to eliminate noise caused by vibration. This system does not use adouble window, and generates two beams, the separation of which can be variedaccording to the characteristics of the grid used. Experimental results are also given.