This paper presents a detailed analysis of the batch kinetic fermentative assays data used to evaluate how glucose concentration influences fermentative H-2 production, including the non-dissociated acid concentration threshold, with the new isolate Clostridium beijerinckii Br21. Fermentative batch assays for hydrogen (H-2) production were conducted with initial glucose concentrations of 5.2, 27.2, 54.4, 97.2, or 154.4mmol/L without pH control. The dry cell mass (X), glucose (S), and product (H-2) concentrations and the pH were monitored for 60h. Increasing initial glucose concentration raised the maximum specific H-2 production rate to values as high as 1.19mmol H-2/mg Xh. However, the H-2 production yield (Y-H2/S) decreased from 2.23 to 0.2mmol H-2/mmol glucose. The initial glucose concentration that provided the best compromise between Y-H2/S and H-2 productivity was 37mmol/L (6.7g/L). The lactic acid, acetic acid, and butyric acid concentrations enhanced at higher initial glucose concentrations, leading to the largest pH decrease. Lower pH favoured non-dissociated acids, which switched the Clostridium metabolism to solventogenesis. The threshold acid concentrations for C. beijerinckii Br21 solventogenesis were non-dissociated butyric acid at 8mmol/L and total non-dissociated acetic and butyric acids at 13mmol/L. Glucose supply and pH control in continuous or fed-batch biorreactors could culminate in higher H-2 productivity and yield by C. beijerinckii Br21.