During preparation of alkyd resin by reaction between isocaprylic acid, phthalic anhydride, glycerol, and pentaerythritol the samples were successively taken away from the batch at different stages of the process and evaluated by analytical and physical ways. Acid numbers, hydroxyl numbers, and saponification numbers were determined by analyses of the samples and number-average molecular weights of alkyds were determined from vapor pressure osmotic measurements of their solutions. The extent of intramolecular esterification reactions (4–5%) was determined from these values. Molecular weight polydispersities in alkyd samples prepared at higher degrees of conversion were estimated by means of GPC measurements, and weight-average molecular weight of the final alkyd resin was calculated by using theoretical relationships derived for nonlinear condensation polymers.