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The presence of fluctuations in the discharge parameters introduces errors into Langmuir probe measurements. Previous work by Garscadden and Emeleus considered the effects of fluctuations in the individual parameters. In this paper a generalized small‐amplitude theory of the effects of these fluctuations is given and also a large signal theory for single‐frequency oscillations. The results of Garscadden and Emeleus can be deduced from these, but additional terms are predicted due to cross modulation occurring when the various fluctuations are correlated. The large signal theory has been checked experimentally in a mercury‐vapor discharge at a pressure of about 1 μ. Further experimental measurements with injected broadband noise signals confirm certain features of the generalized theory. The errors caused by fluctuations are discussed and it is concluded that although electron temperatures may still be measured accurately in their presence, serious errors can be incurred in the measurement of electron saturation current and number density.