This study determined the effects of altering the H⁺ concentration ([H⁺]) during interval training, by ingesting NaHCO₃ (ALK-T) or a placebo (PLA-T), on changes in muscle buffer capacity, endurance performance and muscle metabolites. Pre- and post-training VO_2peak, lactate threshold (LT) and time to fatigue (TTF) at 100% pre-training VO_2peak intensity were assessed in 16 recreationally-active females. Subjects were matched on the LT, randomly placed into the ALK-T (n=8) or PLA-T (n=8) group and performed 8 wk (3 dwk^-1) of 6-12, 2-min cycle intervals at 140-170% of their LT, ingesting NaHCO₃ or a placebo prior to each training session (work matched between groups). Both groups had improvements in muscle buffer capacity (19 v 9%; P < 0.05) and VO_2peak (22 v 17%; P < 0.05) after training, with no differences between groups. There was a correlation between pre-training muscle buffer capacity and percent change in muscle buffer capacity (r = -0.70, P < 0.05). There were greater improvements in the LT (26% v 15%; P = 0.05) and TTF (164% v 123%; P = 0.05) following ALK-T, compared to PLA-T. There were no changes to pre- or post-exercise [ATP], [PCr], [Cr], [La^-]_i or pH_i after training. Our findings suggest that training intensity, rather than the accumulation of H⁺ during training, may be more important to improvements in muscle buffer capacity. The group ingesting NaHCO₃ before each training session had larger improvements in endurance performance, possibly due to a reduced metabolic acidosis during training and a greater improvement in muscle oxidative capacity.