Although xylose is the second most abundant sugar on Earth due to its high presence in lignocellulosic biomass, literature about its fermentation is not as comprehensive as that about glucose fermentation. This study investigated xylose fermentation by anaerobic mixed cultures under different pH, organic loading rate (OLR), hydraulic retention time (HRT), and reactor configuration conditions. Two CSTRs and two SBRs were inoculated with biomass from the same sewage sludge anaerobic digester and operated at 37 ºC. CSTRs were operated for up to 180 days under pH values of 4.5–6.0 and HRTs of 0.5–1 day, whereas SBRs were operated at pH 6 and HRTs of 0.5–1 day, with OLRs of 12 and 24 g COD/L·d. Microbial communities were characterised by 16S rRNA gene amplicon sequencing. Low pH (4.5) favoured Sporolactobacillus and lactate production, whereas pH 5.0–6.0 supported butyrate production with Bifidobacterium dominance. SBRs promoted higher microbial diversity and selectivity toward medium-chain carboxylates, particularly under organic overload. CSTRs showed resilience to hydraulic overload, with shifts in community composition but limited impact on product profiles, indicating the functional redundancy. These results highlight the importance of operational parameters and reactor design in steering microbial communities and optimising carboxylate yields from xylose fermentation.