This methodological study investigated how the distribution of training sessions—massed, equal spacing, and expanding spacing—affects L2 phonetic learning, focusing on Mandarin-speaking learners’ perception of the English /ɛ/–/æ/ contrast. While most previous phonetic training studies have used massed schedules, the current quasi-experimental design revealed that both types of spaced practice significantly outperformed massed practice in terms of immediate gains and long-term retention. Effect sizes in the spaced groups were approximately double those of the massed group. No significant differences emerged between equal and expanding spacing. These findings suggest that distributed practice—regardless of spacing type—can enhance both the magnitude and durability of L2 phonetic learning. Crucially, this study makes it possible to revisit past findings based on massed training paradigms and to consider whether adopting alternative timing schedules could unlock greater learning potential—for instance, by doubling the size and durability of training effects through the use of spaced conditions.
