Styrene (ST)/butyl acrylate (BA) copolymers with average molecular weights as low as (M) over bar(n)=1500 and M(w) = 3000, i.e. co-oligomers, were synthesized thermally at 230-280 degrees C in a continuous stirred tank reactor without any added chain transfer agent. The products were characterized by size exclusion chromatography in conjunction with multiwavelength u.v., as well as by i.r., H-1 n.m.r. and differential scanning calorimetry. The molecular weights of the products decreased with increasing BA content in the monomer feed. The relationship between the residual monomer composition and the product composition can be modelled satisfactorily using a mathematical model based on the instantaneous copolymerization equation and Arrhenius relationships for the reactivity ratios. The devolatilized products varied in form from elastic semisolids through glue-like liquids to viscous liquids.