The plant glucan phosphatases Starch EXcess 4 (SEX4) and Like Sex Four2 (LSF2) apply different starch binding mechanisms. SEX4 contains a carbohydrate binding module, and LSF2 has two surface binding sites (SBSs). We determined KDapp for amylopectin and amylose, and KD for β-cyclodextrin and validated binding site mutants deploying affinity gel electrophoresis (AGE) and surface plasmon resonance. SEX4 has a higher affinity for amylopectin; LSF2 prefers amylose and β-cyclodextrin. SEX4 has 50-fold lower KDapp for amylopectin compared to LSF2. Molecular dynamics simulations and AGE data both support long-distance mutual effects of binding at SBSs and the active site in LSF2.