The effect of varying the C to O atomic ratio on the formation of fullerenes in sooting flames produced by the laser pyrolysis of benzene-based mixtures was investigated. Infrared spectra were recorded on soot toluene extracts and quantification of C-60 and C-70 was performed by high Performance Liquid Chromatography. A strong increase of the fullerenes yield (C-60 + C-70) was observed with decreasing C/O ratio (2.2-1.2). The lowering of the C/O ratio was also found to influence dramatically the ratio of C-60 to C-70 through an increase of the C-60 yield faster than the C-70 one. These two effects can be fully interpreted by a kinetically based argument. A strong increase of the fullerenes content was also observed with decreasing residence time in the reaction flame, and a maximum total yield of 0.23% was reached with high conversion rates (from benzene to soot). Ponderable amounts of C-84 were also quantified.