Anomalous chloride concentration enrichment has been detected in marine sediments comprising methane hydrates (MHs). In this study, we designed an electric circuit system linked to the high-pressure resistance cell in which the chloride ion concentration can be directly measured within reliable accuracy under in situ conditions of the deep-sea floor pressure and temperature. Chloride concentration increased under a fast MH formation rate, but no noticeable concentration change was detected under a relatively low-rate. Furthermore, we suggested that the MH formation rate must be maintained at least similar to 10(2) mol m(-2) yr(-1) so as to efficiently enrich chlorides and retain the acquired chlorinity. The present experimental system dose not fully reflect the relatively minor effective variables such as vertical advections in real system, but the results seem to be sufficient for revealing chloride enrichment phenomena induced by fast MH formation rate with free methane gas. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 322-328, 2012