In an earlier work [H. Bodén and M. Åbom, J. Acoust. Soc. Am. 79, 541–549 (1986)] the influence of errors on two-microphone measurements in ducts without flow has been studied. The aim of this article is mainly to extend the earlier work to include the effects of mean flow and also of attenuation during the sound propagation. First, a short review of the various existing two-microphone methods is made. The errors in the measured input data are then analyzed and special attention is paid to the effects of neglected attenuation, nonideal microphones, and flow noise. The influence of errors on the calculated quantities has been investigated and the conclusions from the earlier work have been extended to the case with flow. It is also shown that the neglect of attenuation between the microphones leads to a low-frequency limit for the applicability of the two-microphone method. Finally, a new technique for measuring the Mach number using a two-microphone method is suggested.