The purpose of this study was to investigate the accuracy and reliability of observational data on spontaneous slips of the tongue. Usually slips are jotted down immediately after being heard but there is no possibility of re-examining the correctness or completeness of this transcription. Four listeners noted down slips in this way ("on-line") and their transcriptions were compared with a transcription obtained by repeated listening to the same tape-recorded discussion. The one-line listeners recorded only about one-third as many slips as were detected by repeated listening, and, even so, about half the items noted as slips proved erroneous. There were also considerable differences between the individual listeners regarding the error units.