Free electrons and H2O2 formed in an optical breakdown plasma are found to directly control the kinetics of [AuCl4](-) reduction to form Au nanoparticles (AuNPs) during femtosecond laser-assisted synthesis of AuNPs. The formation rates of both free electrons and H2O2 strongly depend on the energy and duration of the 800 nm laser pulses over the ranges of 10-2400 mu J and 30-1500 fs. By monitoring the conversion of [AuCl4](-) to AuNPs using in situ UVvis spectroscopy during laser irradiation, the first- and second-order rate constants in the autocatalytic rate law, k(1) and k(2), were extracted and compared to the computed free electron densities and experimentally measured H2O2 formation rates. For laser pulse energies of 600 mu J and lower at all pulse durations, the first-order rate constant, k(1), was found to be directly proportional to the theoretically calculated plasma volume, in which the electron density exceeds the threshold value of 1.8 x 10(20) cm(-3). The second-order rate constant, k(2), was found to correlate with the measured H2O2 formation rate at all pulse energies and durations, resulting in the empirical relationship k(2) approximate to H2O20.5. We have demonstrated that the relative composition of free electrons and H2O2 in the optical breakdown plasma may be controlled by changing the pulse energy and duration, which may make it possible to tune the size and dispersity of AuNPs and other metal nanoparticle products synthesized with femtosecond laser-based methods.