Sine wave replicas of spoken words can be perceived both as nonphonetic auditory forms and as words, depending on a listener's experience. In this study, brain areas activated by sine wave words were studied with fMRI in two conditions: when subjects perceived the sounds spontaneously as nonphonetic auditory forms (“naïve condition”) and after instruction and brief practice attending to their phonetic attributes (“informed condition”). The test items were composed such that half replicated natural words (“phonetic items”) and the other half did not, because the tone analogs of the first and third formants had been temporally reversed (“nonphonetic items”). Subjects were asked to decide whether an isolated tone analog of the second formant (T2) presented before the sine wave word (T1234) was included in it. Experience in attending to the phonetic properties of the sinusoids interfered with this auditory matching task and was accompanied by a decrease in auditory cortex activation with word replicas but not with the acoustically matched nonphonetic items. Because the activation patterns elicited by equivalent acoustic test items depended on a listener's awareness of their phonetic potential, this indicates that the analysis of speech sounds in the auditory cortex is distinct from the simple resolution of auditory form, and is not a mere consequence of acoustic complexity. Because arbitrary acoustic patterns did not evoke the response observed for phonetic patterns, these findings suggest that the perception of speech is contingent on the presence of familiar patterns of spectral variation. The results are consistent with a short-term functional reorganization of auditory analysis induced by phonetic experience with sine wave replicas and contingent on the dynamic acoustic structure of speech.