In plasma chemistry, commercially available systems of combined ion energy and mass analyzers are commonly used for plasma parameter control in processes like plasma supported film deposition or surface modifications. The usual way to measure ion distribution functions by those analyzers is to change the reference potential of the analyzer thereby scanning the distribution function. For interpreting measured ion distribution functions we describe the principle of such a combined ion mass and energy analyzer and calculate the measured ion current and its relation to the ion distribution function. In this way, we show that the measured ion current versus the reference potential of the energy analyzer represents a velocity distribution function versus energy. Furthermore, we discuss in how far it is possible to evaluate from this velocity distribution function the total ion current densities and the energy carried by the ions onto a surface exposed to plasma.