Consideration of the many kidnapping cases in Brazil has led to the conclusion that criminals sometimes try to disguise their voices by speaking with a pen or pencil between their from teeth, a condition which places it in a perpendicular line to the sagittal plane. These settings interact among themselves and also with the speaker's basic articulatory setting yielding acoustic effects that are not easy to interpret. This is because different speech segments will be affected in differing degrees, depending on their greater or lesser susceptibility. The acoustic alterations resulting from the type of disguise discussed can be rather misleading for the forensic phonetician. They can cause him to wrongly establish the speaker's dialectal profile, especially since some relevant cues may be masked. This report examines the formant shifts for Brazilian Portuguese oral vowels which occur when the specified disguise is used. The results show that there are significant differences in the quality of the vowels. Perceptually, the most evident effect is the lowering of the high vowels. It is hoped that the discussion to follow will aid the forensic phonetician in upgrading his speaker identification skills.