Diminutive formation, although highly productive in Spanish, is not uniform across all dialects; and even within a specific dialect there is a certain degree of variation amongst speakers. The Maximum Entropy model (Hayes and Wilson) is a probabilistic theory that captures free variation by assigning weights to different phonotactical constraints that are applied in a formation process. The data used in this study, 650 diminutives collected from each one of six different Spanish native speakers of Porteño (Buenos Aires) Spanish, were processed using the Maximum Entropy model in order to specify the constraints that restricted their formation process. In addition, the interaction of this process with phonological processes was drawn for the particular case of words ending in /s/. The proposed constraints and interaction with phonology achieved a very significant account for the variation of diminutive formation in Porteño Spanish.