The influences of inorganic phosphates, as residuals and/or impurities within starch, on the adhesion of acid-treated cornstarch phosphates to cotton and polyester fibers were investigated for warp sizing. The phosphate impurities considered included sodium dihydrogenphosphate, sodium hydrogenphosphate, sodium pyrophosphate, and pentasodium triphosphate. The starches with a phosphate degree of substitution (DS) range of 0.004-0.021 were phosphorylated by heating acid-treated cornstarch at 125°C for 90 min in a solid-state reaction with pentasodium triphosphate. The adhesion was evaluated in terms of the breaking strength and work-to-break of slightly sized rovings. It was found that the phosphorylation improves the adhesion to both fibers even at a low phosphorylation extent. The phosphate impurities within starch have an adverse effect and reduce the adhesion to both fibers. The adverse effect depends on the type and level of the phosphates, decreasing in the series NaH2PO4 > Na2HPO4 > Na4P2O7 ≈ Na5P3O10. The adhesion enhances as the DS increases and as the free phosphate content decreases. Furthermore, the adverse effect can be offset by the phosphate modification of starch.