Objectives: Loss of intelligibility is a major complaint for patients with speech disorders, as it affects their everyday communication and thus contributes to a decrease in their quality of life. Several tests are available to measure intelligibility, but these tests do not take into account the evaluators’ ability to restore distorted sequences. Due to this ability, the evaluator will tend to recognize words despite phonetic distortions, and speech production deficit can go undetected. The results of these tests therefore overestimate the intelligibility of patients and may mask real functional limitations. We propose a new test which uses a large number of pseudowords in order to neutralize the unwanted perceptual effects that cause this overestimation. The purpose of this test is to measure the speech production deficit. It is not intended to assess the communication deficit. Our objective is to validate this test based on acoustic-phonetic decoding of productions from patients with speech disorders. Materials and Methods: We tested this method with a population of 39 healthy participants and 78 post-treatment patients with cancers of the oral cavity and the oropharynx (HNC patients). Each speaker produced 52 pseudowords taken from randomly generated lists from large common dictionary, each list of 52 pseudowords containing the same number of phonemes. Forty everyday listeners then transcribed these productions. The orthographic transcriptions were phonetized and compared to the expected phonetic forms. An algorithm provided a Perceived Phonological Deviation score (PPD) based on the number of features that differed between the expected forms and the transcribed items. The PPD thus provided a score representing the loss of intelligibility. Results: The 39 participants in the control group demonstrated significantly lower PPD scores compared to the 41 patients with a T1T2 tumor size or compared to the 37 patients with a T3T4 tumor size. The differences between the three groups were significant. If we use the PPD as a predictor to identify patients versus control group subjects, the AUC of the ROC curve is equal to 0.94, which corresponds to an outstanding group separability. A PPD threshold at 0.6 features per phoneme is the boundary between normal and dysfunctional speech. The analysis showed a close correlation between the PPD and a clinical judgment of the disorder severity obtained from experts. Conclusion: This test appears to be effective in measuring the intelligibility of speakers at a phonological level, in particular in the case of head and neck cancers.