The viscosities of the binary mixtures of formic acid-water were measured at 250 with horizontal viscometer under applied pressure.The viscosity increases continuously with increasing concentration of formic acid.The two platforms in the viscosity-composi- Lion curve at mole ratios 1:1 and 1:2 observed by Glagoleva have not been confirmed.For mixtures with formic acid concentration between 55 and 90%,the viscosity varies linearly with the weight fraction of formic acid and can be expressed as η²⁵(cps)=0.633+1.007W.