To address the issue of declining reading achievement in the school where she worked, the author analyzed reading records and written work of primary students attending several district elementary schools over the course of one school year. From that analysis, it appeared that an obstacle to reading success for many children in the third grade was automaticity in the application of the alphabetic principle, specifically vowels. There was a widespread need for intervention, so whole-class intervention in intact classrooms was deemed appropriate. The author developed a whole-class intervention consisting of 60 multisensory word-study lessons for third grade, and designed a causal-comparative research plan in order to evaluate its effectiveness. For three of the measures of achievement, there was a meaningful statistically significant difference between the treatment groups. Data support the effectiveness of the multisensory word-study program as a whole-class intervention in increasing decoding ability, in developing the ability to correctly encode common phoneme-grapheme spelling patterns, and in increasing automaticity in application of the alphabetic principle through word-reading speed while reading in connected text. However, effects of comprehension instruction in the classroom appear to outweigh the effects of code-based instruction. [This article is based on the author's doctoral dissertation, "The Effects of Multisensory Vowel Instruction during Word Study for Third-Grade Students," which received the International Reading Association's Outstanding Dissertation Award for 2007.]