Biological conversion of gaseous compounds (e.g., H-2/CO2) into valuable liquid fuels or chemicals using mixed culture is a promising technology, which could be effectively and efficiently implemented in a membrane biofilm reactor (MBfR) with gas being supplied from inside of membranes. In this study, a model integrating multiple production pathways of fatty acids (including acetate, butyrate, and caproate) was developed and tested using reported mixed culture experimental data from a lab-scale MBfR fed with 60% H-2 and 40% CO2. The uncertainty of the four estimated model parameters was explored by a sensitivity analysis. With the developed model, the impacts of key process parameters (i.e., gas supply and hydraulic retention time (HRT)) on the performance of the MBfR converting H-2/CO2 to fatty acids were then investigated. The results show that a high HRT is imperative for chain elongation to produce a higher proportion of caproate with a higher added value. A proper gas supply should be provided to favour the speciation of biological gas conversion products as well as to fully exploit the conversion capacity of the MBfR. The findings of this work provide useful information for a better understanding and further applications of this MBfR technology for mixed culture syngas fermentation. (C) 2016 Elsevier B.V. All rights reserved.