Small differences in quality may affect the ease with which various speech samples can be understood even though they are all highly intelligible according to articulation tests. Such quality differences were reliably discriminated by a speech‐interference test which determined how easily the speech samples could be heard against a background of interfering speech. The communication quality of a degraded speech sample was measured by the difference,A = T − T0, whereTandT0were the interference thresholds for the degraded and reference speech, respectively. An interference threshold was defined as the value of the signal‐to‐interference intensity ratio where the signal speech was 50% intelligible in the presence of interfering speech.Qmeasures and subjective quality ratings were obtained for speech degraded by low‐ and high‐pass filtering, additive noise, and transmission over a telephone line. TheQmeasures successfully discriminated differences in speech quality, and were monotonically related to the ratings, thereby validating the speech interference test for speech‐quality evaluation.