The photopolymerization activities of three novel perester derivatives of fluorenone and a mono and tetra perester derivative of benzophenone in methyl methacrylate (MMA) and an ethoxylated bis-phenol-A diacrylate have been studied using a combination of gel permeation chromatography (GPC), photodifferential scanning calorimetry (PDSC) and real time Fourier transform i.r. spectroscopy (RTFTIR). Using GPC analysis on the polyMMA samples, both the mono-and tetra-tert-butylperester derivatives of benzophenone are found to give higher weight-(ATw) and number-average molecular weight ()1~,) polymers than those produced using the three fluorenone initiators with the tetra derivative giving the highest values. Both A~ n and A~ w values for the polymers obtained using the fluorenone derivatives were highest for the 4-tert-butylperester-7-nitro derivative followed by the 2-tert-butylperester and then the 2,7-di-tert-butylperester derivative. Whilst the former fluorenone initiator has the highest extinction coefficients in the near u.v. region, the latter exhibits a higher degree of self termination. The presence of a tertiary amine co-synergist (N-methyldethanolamine) is found to have no significant influence on k~~ or h7 W of the polymers produced with the three fluorenone derivatives. Using PDSC, the efficiencies of the initiators are found to be dependent upon their structure and concentration. For some of the fluorenone photoinitiators, maximum photoinitiation efficiency occurs at 0.25% w/w above which screening and self-termination appear to dominate the photopolymerization with the bis-tert-butylperester derivative being the most efficient. For the two peresters of benzophenone, photopolymerization efficiency increases with increasing initiator concentration and no self-termiantion is observed up to 0.75ยฐ/, w/w concentration. PDSC is concluded to be a valuable comparative method for reinforcing other methods of characterization. The results obtained using RTFTIR were similar in many respects to those obtained using PDSC with the bis-tert-butylperester derivative of fluorenone being the most effective initiator.