► Power degradation occurred in a direct methanol fuel cell (DMFC) with a plastic tube was investigated. ► Di-(2-ethylhexyl) phthalate (DEHP), a plasticizer, was detected in the circulated methanol solution. ► The degradation in performance by DEHP was confirmed. ► DEHP would affect the methanol transport through the PCP. The cause of the power degradation that occurred in a direct methanol fuel cell (DMFC) with a plastic tube for the connection was investigated. The DMFC had a unique electrode structure with a porous carbon plate in order to control the methanol crossover, and then operated with a 99.8wt% methanol solution. A certain amount of di-(2-ethylhexyl) phthalate (DEHP), a plasticizer, was detected in the circulated methanol solution. The degradation in performance by DEHP was confirmed by comparing the performances with and without DEHP addition in the methanol. It was postulated that the methanol transport through the PCP was significantly affected by the addition of DEHP.