The ability of a likelihood ratio processor to classify similar spherical underwater targets using quadrature components of their acoustic echoes was measured under highly controlled conditions. The five targets used were 7‐in.‐diam, 5‐in.‐diam, and 3‐in.‐diam solid aluminum spheres; a 5‐n.‐diam styrofoam sphere; and a 5‐in.‐diam hollow aluminum sphere. A pulsed sinusoid and a linear FM signal were transmitted, both with the same bandwidth. The performance of the likelihood ratio processor for a specified signal‐to‐noise (S/N) ratio (peak amplitude of echo to rms of noise amplitude) was portrayed with a Receiver Operating Characteristic (ROC) curve. For the pulsed sinusoid with known epoch, the performance was satisfactory for a majority of the target pairs when the S/N ratio was 0 dB or larger. For the pulsed sinusoid, it was found that the S/N ratio was a sensitive parameter that affected performance, and that an uncertainty in epoch over a cycle of the carrier resulted in the same degradation in performance as a 6‐dB decrease in the S/N ratio. The performances obtained with the likelihood ratio processor for the linear FM transmission when the S/N ratio was −6 dB were of the same order as, but better than, that obtained for the sinusoidal pulse when the S/N ratio was 0 dB. Equivalent performances were obtained for both transmissions when the signal energies to mean‐square‐of‐noise amplitudes of the messages for both transmissions were equal.