The ratio k(cat)/K(M)--often referred to as the "specificity constant"--is a useful index for comparing the relative rates of an enzyme acting on alternative, competing substrates. However, an alternative description, "catalytic efficiency", is frequently used, and on occasions misused, to compare the reactivity of two enzymes acting on the same substrate. Here, we highlight the pitfalls in using k(cat)/K(M) to compare the catalytic effectiveness of enzymes.