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This paper presents a method for estimating the in-phase/quadrature-phase (I/Q) mismatch in low-intermediate frequency receivers for orthogonal frequency division multiplexing (OFDM) signals in the presence of carrier-frequency offset. The proposed estimator operates on the repetitive training sequence of the OFDM packet preamble. To derive a practical maximum-likelihood estimator, an invertible mapping operation is applied on the observation vectors, so that a closed-form expression of the likelihood function can be obtained. The proposed estimator is shown to achieve the Cramer Rao lower bound at high image signal power levels. The error variance of the estimator is analyzed, which closely matches simulation results. The uncoded bit-error-rate is shown via simulation to be approximately that of an ideal receiver without I/Q mismatch.