Objective: The purpose of this in vitro study was to investigate the effect of isopropyl alcohol and unfilled resin on the flexural strength of incrementally layered resin-based composite beams. Method and Materials: Five groups of 30 composite beams were fabricated in a 2 3 2 3 24-mm Teflon split-mold: group 1 (control group), a homogeneous resin composite beam; group 2, 2 increments with an air-inhibited layer between the increments; group 3, 2 increments with a Mylar strip placed against the composite interface during polymerization of the first increment; group 4, 2 increments with the first increment contoured with an isopropyl alcohol (70%) lubricated instrument; and group 5, 2 increments with the first increment contoured with unfilled resin lubricating the application instrument. Each increment was polymerized for 40 seconds on both front and back surfaces. The flexural strength of the composite specimens was determined using 4-point bending in an Instron universal testing machine. Results: The mean flexural strength in MPa and standard deviation were as follows: group 1: 142.5 (17.3); group 2: 98.2 (15.2); group 3: 105.9 (16.3); group 4: 103.4 (19.4); and group 5: 106.1 (21.3). One-way analysis of variance detected a statistically significant difference among the groups (P .001). Tukey's HSD post hoc tests determined that only group 1 was significantly different from the other 4 groups. Conclusion: The use of isopropyl alcohol and unfilled resin as instrument lubricants for incremental composite buildup seems to have little effect on the flexural strength of light-cured composite. Keywords: alcohol, composite, flexural strength, incremental layering technique, lubricant