Mono- and polyvalent galactosides have been investigated with respect to their binding to the plant lectin Ricinus communis agglutinin 120 (RCA 120). Thermodynamic parameters (Ka, ΔG, ΔH, ΔS and n) have been determined by isothermal titration calorimetry (ITC) and kinetics of binding (ka and kd) measured by surface plasmon resonance (SPR). ITC analysis using a single set of sites model found a non-statistical increase in avidity with increasing valency with the largest ligand displaying a greater than 20-fold increase in Ka compared to its monomeric precursor after correction for valency; binding was found to be enthalpically driven. SPR analysis supports the avidity increase but values of Ka observed were up to 100-fold greater than those measured by ITC. The large discrepancy between the two measurements is rationalized by the polyvalent-polyvalent interaction that is measured by SPR.