Biomass fermentation technologies offer alternative methods to produce platform chemicals that currently originate from fossil sources. This research showed that an enriched microbiome was capable to produce iso-butyric acid via methanol based chain elongation of acetate. A long term continuous reactor experiment showed that the selectivity for isobutyrate (i-C4) and/or n-butyrate (n-C4) could be reversibly adjusted by changing the reactor pH. A reactor pH of 6.75 led to formation of (carbon per total carbon of products) of 0.78 n-C4 and 0.024 i-C4, whereas a reactor pH of 5.2 led to a selectivity of 0.25 n-C4 and 0.63 i-C4 . This shift in product spectrum was also represented by a shift in microbial composition. The results suggest that an Eubacterium genus is responsible for the formation of n-C4, whereas a Clostridium luticellarii strain is responsible for the formation of a mixture of i-C4 and n¬¬-C4. The formation of n and i-C4 at a low pH was observed to be coupled according to the thermodynamics of isomerization. At a reactor pH of 5.5 and 5.2 the product ratio of i-C4:n-C4 approached the ratio of 0.684 i-C4 : 0.316 n-C4, which is the theoretical ratio that would be achieved when i-C4 and n-C4 would be formed at amounts determined by the equilibrium of isomerization. Various batch experiments at pH 5.5 and 5.2 confirmed that addition of either n-C4 or i-C4 at the start of the batch would immediately lead to the formation of the other butyrate component. Moreover, batch experiments performed at pH 6.5 immediately produced mainly n-C4 and led to the development of a completely different microbiome. The imposed pH as a strong selective pressure can immediately facilitate changes in product selectivities for n-C4 and i-C4 during methanol based chain elongation of acetate.