Much effort has been expended to define individual instrumental variables and chemical requirements for kinetic assays. In this paper, these variables are grouped together to demonstrate the relationship between characteristics of the instrument system and final results for kinetic enzyme analyses. Tolerances on instrumental variables are reviewed in terms of these final results, and summarized and interpreted as they effect the results of kinetic assays. We have evaluated the effects on absorbance change as they are related to tolerances for absorbance, time, sample volume, reagent volume, temperature, and path-length. Specifications of spectrophotometric instruments in terms of linearity, accuracy, wavelength adjustability and accuracy, stray light, noise, and drift are also reviewed as to their effects on absorbance change. Generalized data are presented and are then related, as an example, to measurement of aspartate aminotransferase activity.