Purpose: To measure the softening via swelling of a composite resin by evaluating its surface hardness after the application of four monomer systems. Materials and Methods: A total of 40 specimens made of a hybrid composite resin were evaluated. Four monomer systems were used as surface-conditioning primers during five exposure times. Surface microhardness measurements (n = 8) were made after application of the monomer systems. Results: In all exposure times, Scotchbond Universal showed the highest influence on the composite resin surface softening (P .05), followed by Stick Resin (P .05). Conclusion: Surface softening of composite resins might be achieved with monomer systems, especially with Scotchbond Universal, which may have a positive influence when repairing aged composite resin restorations.