Background: Chlorhexidine is well known for its antiplaque effect. However, the mouthrinse based chlorhexidine antiplaque efficiency may vary according to the formulation of the final product. The aim of the present study was to compare anti-plaque effectiveness of two commercial mouthrinses: 0.12 % Chlorhexidine alcohol base (CLX-A) versus a diluted 0.1% Chlorhexidine non-alcohol base with 0.1% of Formaldehyde (CLX-F). Material and Methods: the study was a seven day randomized, double-blind, placebo-controlled trial including 30 volunteers. At the start, all participants received a dental prophylaxis. Over 7 days experimental non-brushing period, during which subjects abstained from all forms of mechanical oral hygiene, one group test rinsed twice daily with 15ml of an alcohol base 0.12% Chlorhexidine mouthrinse. The second group test used 15ml of alcohol free 0.1% Chlorhexidine mouthrinse base 0.1% formaldehyde twice daily. The negative control group used a placebo. Plaque indexes were recorded in all volunteers prior to treatment at Day 0, 1 and 7. Results: After 7 days, the mean plaque index for the first group was 0.76±0.38 compared with a mean plaque index of 1.43±0.56 for the second group. The difference in plaque scores between the groups was statistically significant. Conclusion: the results of this study showed that rinsing with an alcohol base 0.12% Chlorhexidine mouthrinse is significantly different from rinsing with an alcohol free 0.1% Chlorhexidine mouthrinse on plaque inhibition.