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A comparative biochemical characterization is described of two competing enzymes in the production of flavoring 5′-ribonucleotides, barley malt sprouts 5′-phosphodiesterase (5′-PDE) and phosphomonoesterase (PME). Fractionation of these two enzymes and partial purification of 5′-PDE were achieved by a combination of thermal treatments and precipitation with acetone. With synthetic substrates, under standard assay conditions, 5′-PDE and PME had maximum activities at pH 8.9, 70°C and 55°C, and K_m of 0.26 mM and 0.19 mM, respectively. In the presence of 10 mM Mg²⁺ ions, barley malt sprouts 5′-PDE was activated by up to 160% of the original activity, while PME was inhibited. Zn²⁺ activated PME by up to 125 % of the original activity. Both enzymes were moderately inhibited after addition of Cu²⁺, Co²⁺, Ca²⁺, and Mn²⁺ ions (10 mM), but, significantly, by addition of the chelating agent EDTA …