Bulky, liquid dimethacrylates, designated DIM-GMA and PGA-GMA, derived from the reaction of dimer acid (DIM) and a polyglycolic acid (PGA) respectively, with two equivalents of glycidyl methacrylate (GMA) were recently prepared and characterized. Objective: Determine the effect on degree of vinyl conversion (DC) and polymerization shrinkage (PS) of using these high molecular weight dimethacrylates as diluent comonomers, rather than triethylene glycol dimethacrylate (TEGDMA), with ethoxylated bisphenol A dimethacrylate (EBPADMA) in similar glass-filled composites. Materials and Methods: DIM-GMA (Mw ~ 854) and PGA-GMA (Mw ~ 884) were blended with equal masses of EBPADMA, photoactivated for visible light photopolymerization and admixed with 0.847 mass fraction of silanized Z100 glass (3M). As a control a similar composite was prepared with an equivalent amount (~ 0.16 mass fraction) of TEGDMA (Mw=286) as the diluent comonomer for EBPADMA (Mw ~ 540). DC was assessed by near infrared spectroscopy (vinyl absorption = 6165 cm-1) and PS by mercury dilatometry. Results: Mean values are given as percent with standard deviation in parentheses (number of specimens = 3). For the DIM-GMA composite, %DC after 24 h at 23 °C was 68.1 (1.9), PS = 1.53% (0.066); PGA-GMA, DC = 73.0 (3.2), PS = 1.59% (0.045); TEGDMA, DC = 56.4 (2.1), PS = 1.81% (0.053). Conclusion: Both composites formulated with the high Mw, flexible comonomers gave significantly higher DC and lower PS mean values than the control composite formulated with an equivalent amount of TEGDMA. Support from NIDCR/NIST Interagency Agreement No. Y1-DE-1021-02.