In this work, the colloidal stability of a pharmaceutical ethylcellulose latex, Aquacoat, is analyzed by means of turbidity measurements. The effects of both electrolyte (KCI, CaCl2 and LaCl3) concentration and pH on the stability are considered, and it is found that at pH 4 (6), the critical coagulations concentrations (c.c.c.) are 70 (150) mM KCI, 12 (20) mM CaCl2, and 1 (1) mM LaCl3. The results are interpreted in terms of both classical and extended versions of the DLVO theory of the stability of colloids; the Hamaker constant, A, characteristic of the van der Waals interaction between the particles in suspension, is calculated from measurements of the surface free energy of the polymer, based on contact angle measurements of selected liquids on polymer pellets. Concerning the electrostatic contribution to the total interaction energy between the polymer spheres, it is calculated from zeta potential data deduced from electrophoretic mobility of the latex suspensions. The application of the so-called extended DLVO theory includes not only van der Waals and electrostatic interactions, but also acid-base or structural ones, estimated using a recent model of the surface free energy of solids. Using the well-known Fuchs equation, we have computed the stability ratio W of the suspensions, using both the classical and extended DLVO theories, for the different experimental conditions. The results show that, when the electrolyte is KCI, the classical model correctly predicts the c.c.c. values, although the slopes of the log W concentration trends are different from the experimental ones. The extended theory underestimates the c.c.c.’s and gives a better approximation to the slopes. In the case of both CaCl2 and LaCl3, the best agreement is always found with the extended theory. It is suggested that consideration of ionic strength effects on the acid-base characteristics of the solid could improve the extended DLVO results.