Abstract An extended set of equations which describe coupled arginase-urease reactions is presented. The mathematical treatment leads to the development of new equations which relate the lag-time required for the concentration of urea to reach a defined fraction of its steady-state concentration to the kinetic parameters of the enzymes, when the steady-state concentration of urea is small compared to its Michaelis constant value, the coupled arginase-urease reaction was monitored with a surface acoustic wave (SAW) enzyme sensor system, to couple the biochemical selectivity of enzymes with the sensitivity of SAW sensors. The proposed theoretical expressions were verified experimentally. The kinetic parameters of urease and arginase (extracted directly from bovine liver) were examined under theoretical guidance. Dependence of the lag-time of the coupled enzyme reaction on the concentrations of urease and arginase is also described.