Water　flooding　of　the　flow　channels　is　one　of　the　critical　issues　to　the　design　and　operation　of　proton　exchange　membrane　fuel　cells　(PEMFCs).　The　liquid　water　and　total　pressure　drop　characteristics　both　in　the　anode　and　cathode　parallel　flow　channels　of　an　operating　PEMFC　were　experimentally　studied.　The　gas/liquid　two-phase　flow　both　in　the　anode　and　cathode　flow　channels　was　observed,　and　the　total　pressure　drop　between　the　inlet　and　outlet　of　the　flow　field　was　measured.　The　effects　of　cell　temperature,　current　density　and　operating　time　on　the　total　pressure　drop　were　investigated.　The　results　indicated　that　the　total　pressure　drop　in　the　flow　channels　mainly　depends　on　the　resistance　of　the　liquid　water　in　the　flow　channels　to　the　gas　flow,　and　the　different　flow　patterns　distinguish　the　total　pressure　drops　in　the　flow　field.　Clogging　by　water　columns　result　in　a　higher　pressure　drop　in　the　flow　channels.　The　total　pressure　drop　measurement　can　be　considered　as　an　in　situ　diagnoses　method　to　characterize　the　degree　of　the　flow　channels　flooding.　The　liquid　water　in　the　cathode　flow　channels　was　much　more　than　that　in　the　anode　flow　channels.　The　pressure　drop　in　the　cathode　flow　channels　was　higher　than　that　in　the　anode　flow　channels.　During　the　fuel　cell　operation,　the　cell　performance　decreased　gradually　and　the　pressure　drop　both　in　the　anode　and　the　cathode　flow　channels　increased.　The　rate　of　flooding　at　the　cathode　side　reached　49.56%　under　experimental　conditions　after　78　min　of　operation.　However,　it　was　zero　at　the　anode　side.　(c)　2006　Elsevier　Ltd.　All　rights　reserved.