The flashpoint of a viscous flammable liquid measured by the Tag closed tester decreases generally and linearly as the rate of rise ofthe sample tempemture increases,especially when the multi-component Iiquid contains small amounts of a component with a relatively low flashpoint・This is due to a temperature inequality,which depends on both the kinematic viscosity and on the rate of rise in temperature,throughout the sample in the test cup,The Tag closed tester,in accordance with the standard test method,1eads to inevitable error to be more than2〔℃〕for the liquids whose kinematic viscosity exceed!〔cSt〕at their flashpoint.When the flashpoint,extrapolated from the regression line of the Tag nashpoint empiricallyc・rrelatedwiththerate・friseintemperatureatzer・,isequalt・theSetaflashp・intwithintherange・f experimental error,these are real flashpoints in equilibrium between liquid and vapour、The Pensky-Martens closed tester is not always suitable fQr viscous flammable liquids,since its flashpoiut is usually measured at approximately 4.3℃ below the real flashpodnt.