Xyloisosaccharinic acid is one of the major degradation products formed during the alkali catalysed hydrolysis of hemicelluloses. In acidic solution xyloisosaccharinic acid undergoes an acid catalysed lactonisation to generate xyloisosaccharino-1,4-lactone. We report here the solution phase properties of xyloisosaccharinic including measurement of its aqueous pKa (3.00 ± 0.05) using 13C NMR methods. We also report rate constants for the acid catalysed lactonisation, klact(D20), of xyloisosaccharinic acid and the results of our investigations of the kinetics of hydrolysis of xyloisosaccharino-1,4-lactone at acidic and basic pHs. The second-order rate constants for the hydrolysis reactions kHO- (25 M-1 s-1) and kD+ (4.13 E-4 M-1 s-1).