Recent molecular dynamics (MD) simulations of proteins have suggested that common force fields overestimate the strength of amino acid interactions in aqueous solution. In an attempt to determine the causes of these effects, we have measured the osmotic coefficients of a number of amino acids using the AMBER ff99SB-ILDN force field with two popular water models, and compared the results with available experimental data. With TIP4P-Ew water, interactions between aliphatic residues agree well with experiment, but interactions of the polar residues serine and threonine are found to be excessively attractive. For all tested amino acids, the osmotic coefficients are lower when the TIP3P water model is used. Additional simulations performed on charged amino acids indicate that the osmotic coefficients are strongly dependent on the parameters assigned to the salt ions, with a reparameterization of the sodium/carboxylate interaction reported by the Aksimeritiev group significantly improving description of the osmotic coefficient for glutamate., For five neutral amino acids, we also demonstrate a decrease in solute solute attractions using the recently reported TIP4P-D water model and Using the KBFF force field. Finally, we show that for four two residue peptides improved agreement With experiment can be achieved by rederiving the partial charges for each peptide.