Abstract Microbial fermentation of non-digestible carbohydrates in the pig's large intestine induces a shift of N excretion from urea in urine to bacterial protein in faeces. Experiments were carried out to measure the mineral N incorporation by the pig intestinal microflora using 5 purified carbohydrates in a gas-test: starch (S), cellulose (C), inulin (I), pectin (P) and xylan (X). Fermentation kinetics was modelled. N source in the buffer solution was replaced by 15 N labelled NH 4 Cl. The bacterial N fixation was determined at mid-fermentation, measuring 15 N incorporation into the solid phase of the buffer. The bacterial N fixation was higher ( P − 1 ) and C (0.074 h − 1 ) showed lower half time fractional rate of degradation than S (0.153 h − 1 ), P (0.133 h − 1 ) and X (0.104 h − 1 ). The insoluble dietary fibre content of the substrates was negatively correlated to bacterial N fixation ( r = − 0.957, P = 0.011). The high crude protein content of P (32.5 mg g − 1 DM) might explain the lower impact of this substrate on bacterial N fixation, despite its rapid fermentation. Beside the proportion of insoluble fibre, the N content and the rate of fermentation seem to be the major factors influencing bacterial protein synthesis. Further studies including ingredients with variable content of indigestible protein and mean retention time in the pig's intestines are necessary.