A new chemical kinetics mechanism for simulating urban and regional photochemistry has been developed and evaluated. The mechanism, called the Carbon Bond Mechanism IV (CBM-IV), was derived by condensing a detailed mechanism that included the most recent kinetic, mechanistic, and photolytic information. The CBM-IV contains extensive improvements to earlier carbon bond mechanisms in the chemical representations of aromatics, biogenic hydrocarbons, peroxyacetyl nitrates, and formaldehyde. The performance of the CBM-IV was evaluated against data from 170 experiments conducted in three different smog chambers. These experiments included NOx-air irradiations of individual organic compounds as well as a number of simple and complex organic mixtures. The results of the evaluation indicate substantial improvement in the ability of the CMB-IV to simulate aromatic and isoprene systems with average overcalculation of ozone concentrations of 1% for the aromatic simulations and 6% for the isoprene simulations. The mechanism also performed well in simulating organic mixture experiments. Maximum ozone concentrations calculated for 68 of these experiments were approximately 2% greater than the observed values while formaldehyde values were low by 9%. ¿ American Geophysical Union 1989