In this paper, two types of acrylics, polyacrylate (PA), a copolymer of methyl methacrylate, acrylic acid, and others, and polyurethane-modified PA (PU/PA), are investigated in their interactions with inorganic phases in polymer-modified cement-based mixtures and in their mechanisms in improving both mechanical properties and durability of cement-based materials. In these investigations, the total organic carbon content (TOC) method is used to study the adsorption of polymer particles in mixtures. An aqueous tube test, x-ray diffraction (XRD), thermal gravity analysis (TGA), and Fourier transform infrared spectroscopy (FTIR) are employed to study the possible chemical reactions between the latexes and cement hydrates. Scanning electron microscopy (SEM) and energy-dispersive x-ray analysis (EDX) are used to observe the film formation and the final morphology of PA- and PU/PA-modified mortars. PU/PA latex is found to be relatively stable, and the behaviors of PU/PA-modified mixtures can be explained by Ohama's model. On the other hand, molecules of PA latex are very active in cement-based mixtures. Moreover, the normal assumption that polymer particles are uniformly dispersed in aqueous phase is invalid in PA-modified mixtures. Chemical reactions occur between PA latex and cement hydrates, especially calcium hydroxide. In the final comatrix of PA-modified mortar, no PA film with high purity can be distinguished by SEM/EDX. Based on the findings, a new model with a four-step procedure is proposed to explain the behaviors and properties of PA latex-modified cement-based materials. © 2011 American Society of Civil Engineers.