We have employed a theoretical equation of state for the calculation of the properties of real quadrupolar fluids whose interactions can be represented by a square-well with an embedded point quadrupole. This equation has only four disposable parameters: depth, range, diameter, and quadrupole moment. With this simple four-parameter equation we are able to predict the thermodynamic properties of carbon dioxide and nitrogen with good accuracy. This equation is able to reproduce the experimental (vapour + liquid) saturation curve and saturation pressures except near the critical region. Very good predictions of the speed of sound are obtained for both substances in the gaseous region. Molar isochoric heat capacities predictions for carbon dioxide in the liquid region are also presented.