Objective: Sodium hypochlorite (NaOCl) is an important tool in root canal disinfection although it is well known that the shelf-life of NaOCl is limited. In this study, NaOCl solutions that were collected from dental practices and were intended for endodontic irrigation were investigated to see whether they contained the expected concentration of free available chlorine. Method and Materials: NaOCl solutions were collected from dental practices. The concentration of available chlorine per sample was determined with iodometric titration and the pH was measured. Each participating dentist completed a questionnaire that requested data on a range of issues relating to the assumed concentration of NaOCl and handling of the sample. Results: Eighty-four samples with questionnaires were received. NaOCl was purchased from supermarkets and drugstores (36%), dental suppliers (48%), or pharmacies (16%). The median expected concentration was 2% (n = 36). On average, 27% less available chlorine was measured than the dentist assumed was in the sample (P .001). Fifteen percent of samples contained less than 1% available chlorine, which is needed for tissue dissolution and disinfection. The average pH was 11.5. Conclusion: The greatest differences in concentrations were found in NaOCl sourced from supermarkets or drugstores. Future studies should elucidate the cause of this discrepancy. Clinical relevance: In the meantime it is recommended to purchase NaOCl from professional suppliers, because this group showed the most reliable content of free available chlorine. Keywords: bleach, endodontic irrigation, pH, sodium hypochlorite