A high-vacuum falling-ball viscometric method has been used to measure the equilibrium constant for the aggregation of free polystyrylanions in ethylbenzene, The rate of the transfer reaction between polystyryllithium and ethylbenzene has also been measured. All measurements have been made in the absence of monomer under conditions relevant to large-scale polymerization processes, i.e. with solutions containing a high weight fraction of polyanions (approximately 15-70% w/w) and at temperatures in the region of 70 degrees C. The equilibrium constant (K-d) for the aggregation of polystyrylanions was found to be in the range 2 x 10(-3) mol1(-1) to 3 x 10(-2) for polystyrylanions of Mn 30 to 161 kgmol(-1) at 70 degrees C. The chain-transfer constant (k(tr)) determined at 70 degrees C over a range of polymer molecular weights and concentrations is (2.21 +/- 0.05) x 10(-5) 1mol(-1) s(-1). A number of assumptions have been made in order to arrive at the constants which are reported, including, (i) some concerning the viscosity behaviour of concentrated polymer solutions, (ii) that the polymers studied are essentially monodisperse, (iii) that polystyrylanions are two-fold associated in ethylbenzene, and (iv) that only the free polystyrylanions engage in the transfer reaction.