Abstract: An alkenylsuccinic anhydride type paper sizing agent composition which causes little contamination of a paper machine and exhibits an excellent sizing effect is provided.The paper sizing agent composition contains addition reaction products obtained from an addition reaction between a straight-chain internal olefin comprising 16 to 20 carbon atoms and maleic anhydride, any unreacted olefin and maleic anhydride having been removed from said addition reaction products; said addition reaction products comprising as a main component alkenylsuccinic anhydride (1:1 adduct) represented by formula (I): ##STR1## wherein both of R and R' are alkyl groups or either one of R and R' is an alkyl group while the other is a hydrogen atom; and a 1:2 adduct (2 moles maleic anhydride per mole of olefin), and a decarbonated product of said 1:2 adduct in a total amount, based on said addition reaction products, of from about 7 to about 12 mol%.