Purpose: Resonance tube phonation in water (RTPW) or in air is a voice therapy method successfully used for treatment of several voice pathologies. Its effect on the voice has not been thoroughly studied. This investigation analyzes the effects of RTPW on collision and phonation threshold pressures (CTP and PTP), the lowest subglottal pressure needed for vocal fold collision and phonation, respectively. Method: Twelve mezzo-sopranos phonated into a glass tube, the end of which was placed under the water surface in a jar. Subglottal pressure, electroglottography, and audio signals were recorded before and after exercise. Also, the perceptual effects were assessed in a listening test with an expert panel, who also rated the subjects' singing experience. Results: Resonance tube phonation significantly increased CTP and also tended to improve perceived voice quality. The latter effect was mostly greater in singers who did not practice singing daily. In addition, a more pronounced perceptual effect was found in singers rated as being less experienced. Conclusion: Resonance tube phonation significantly raised CTP and tended to improve perceptual ratings of voice quality. The effect on PTP did not reach significance.