Entrainment has played a crucial role in analyzing marital couples interactions. In this work, we introduce a novel technique for quantifying vocal entrainment based on Principal Component Analysis (PCA). The entrainment measure, as we define in this work, is the amount of preserved variability of one interlocutor’s speaking characteristic when projected onto representing space of the other’s speaking characteristics. Our analysis on real couples interactions shows that when a spouse is rated as having positive emotion, he/she has a higher value of vocal entrainment compared when rated as having negative emotion. We further performed various statistical analyses on the strength and the directionality of vocal entrainment under different affective interaction conditions to bring quantitative insights into the entrainment phenomenon. These analyses along with a baseline prediction model demonstrate the validity and utility of the proposed PCA-based vocal entrainment measure.