This series of three papers addresses the representation of secondary organic aerosol (SOA) in atmospheric models. SOA forms when gas-phase organic species undergo oxidation, leading to products of sufficiently low vapor pressure that can partition between the gas and aerosol phases. The present paper, part 1, is devoted to the development of a gas-phase atmospheric chemical mechanism designed to represent ozone chemistry as well as formation of individual organic oxidation products that are capable of forming SOA. The ozone chemistry in the mechanism draws upon the recent work of Stockwell et al. <1997> and Jenkin et al. <1997> and SAPRC-97 and SAPRC-99 (available from W.P.L. Carter at http://helium.ucr.edu/~carter/). The mechanism is evaluated in the three-dimensional California Institute of Technology (CIT) model <Meng et al., 1998> by simulating gas-phase concentrations in the South Coast Air Basin (SoCAB) of California over the period 27--29 August 1987. Total predicted concentrations of gas-phase SOA compounds are compared with levels of SOA that have been inferred on the basis of ambient organic aerosol measurements during this period. These predicted concentrations indicate that the total gas-phase potential of SOA-forming compounds can account for observed aerosol concentrations. Part 2 develops a thermodynamic gas--aerosol partitioning module, and part 3 presents a full three-dimensional simulation of gas and aerosol levels in the SoCAB during a 1993 episode.