MM4 force-field calculations have been extended to carboxylic acids, esters, and lactones. The related parameters were determined mainly by fitting to available experimental data and high-level ab initio calculations on simple molecules. The structures of the training-set molecules were well-reproduced with a rms errors of 0.005 Angstrom for bond lengths and 0.8degrees for bond angles. The moments of inertia for 12 simple molecules of the title class were fit to the experimental (microwave) values with an overall rms error of 0.32%. The vibrational spectra were significantly improved from previous force-field calculations, with a rms error of 19 cm(-1) for 74 frequencies of 5 compounds. The conformational equilibria and rotational barriers were also fit, approximately within experimental error. Although the calculated heats of formation for the carboxyl acids and esters were also improved relative to values in earlier work (MM3), the standard deviation of the heats for 30 selected,compounds is still high (0.67 kcal/mol) compared to those for other classes of compounds previously studied with MM4.