Vowel devoicing is a phenomenon that is reported to occur in many languages such as Japanese, Parisian and Montreal French, Turkish and English. This paper investigates vowel devoicing in Korean. A devoiced vowel does not exhibit characteristic vocal tract resonances, and instead is realized as a long interval of aspiration or frication following consonant release, resulting in non-distinct segment boundaries between devoiced vowels and adjacent voiceless consonants. This paper examines temporal and spectral evidence of devoiced vowels and, among other findings, reveals that in Korean devoiced high vowels are not segmentally deleted but phonetically masked, suggesting that vowel devoicing results from the overlap of glottal gestures. This paper also examines the effect of the preceding consonant place and manner, and the height and front/backness of vowels on devoicing.