NOVELTY - An organic vapor fuel cell comprises a membrane electrode assembly (8); fuel permeation-controlling member (28) positioned in front of anode (10), and substantially impermeable to an organic fuel and/or water at or below an ambient temperature, but being permeable to the organic fuel and/or water at a temperature higher than an activation temperature; heater in heat-supplying relation to the fuel permeation-controlling member to activate fuel permeation through the member on demand; and fuel supply for providing the organic fuel and water as a feeding fuel to the permeation-controlling member.    USE - The fuel cell, e.g. direct methanol fuel cell (DMFC) and direct ethanol vapor fuel cell (DEFC), is used for powering small vehicles and portable devices such as notebook computer, personal digital assistant, mobile phone or digital camera.    ADVANTAGE - The fuel cell is compact and lightweight, with reduced fuel crossover and improved fuel utilization efficiency.    DETAILED DESCRIPTION - An organic vapor fuel cell comprises a membrane electrode assembly, comprising (i) a proton exchange membrane (14) having a front face and a rear face, (ii) an anode being coupled to the front face with the front face and the anode defining an interface to accommodate an anode electro-catalyst, and (iii) a cathode (12) being coupled to the rear face with the rear face and the cathode defining an interface to accommodate a cathode electro-catalyst. A fuel permeation-controlling member is positioned in front of the anode, and is substantially impermeable to an organic fuel and/or water at or below an ambient temperature, but being permeable to the organic fuel and/or water at a temperature higher than an activation temperature to deliver a permeated fuel fluid to the anode. A heater is in heat-supplying relation to the fuel permeation-controlling member to activate fuel permeation through the member on demand. A fuel supply accommodates the organic fuel and water, separately or as a mixture, and provide the organic fuel and water as a feeding fuel to the permeation-controlling member. INDEPENDENT CLAIMS are also included for:    (1) electrically connected fuel cells (in series or in parallel), at least one of the fuel cells being the fuel cell as above; and    (2) planar fuel cell system comprising a membrane electrode assembly, comprising one solid electrolyte sheet having multiple anodes disposed on only one major side of the solid electrolyte sheet and corresponding cathodes disposed on only an opposing major side of the solid electrolyte sheet; fuel permeation-controlling member; heater; fuel supply to deliver the organic fuel and water to the permeation-controlling member; and multiple current collectors each having an interconnect; where each anode or each cathode has one current collector, the interconnect being arranged to provide electron transfer between anodes and cathodes.    DESCRIPTION OF DRAWING(S) - The figure is a cross sectional view showing the structure of the components of a fuel cell that operates on a fuel vapor.    Membrane electrode assembly (8)    Anode (10)    Cathode (12)    Proton exchange membrane (14)    Fuel chamber (22)    Fuel permeation-controlling member (28)    Tab (32)