Microbial community profiles of recently formed hot spring systems ranging in temperatures from 57 ° C to 100 ° C and pH values from 2 to 4 in Hveragerði (Iceland) were analyzed with PhyloChip G3 technology. In total, 1173 bacterial operational taxonomic units (OTUs) spanning 576 subfamilies and 38 archaeal OTUs covering 32 subfamilies were observed. As expected, the hyperthermophilic (∼100 ° C) spring system exhibited both low microbial biomass and diversity when compared to thermophilic (∼60 ° C) springs. Ordination analysis revealed distinct bacterial and archaeal diversity in geographically distinct hot springs. Slight variations in temperature (from 57 ° C to 64 ° C) within the interconnected pools led to a marked fluctuation in microbial abundance and diversity. Correlation and PERMANOVA tests provided evidence that temperature was the key environmental factor responsible for microbial community dynamics, while pH, H2S, and SO2 influenced the abundance of specific microbial groups. When archaeal community composition was analyzed, the majority of detected OTUs correlated negatively with temperature, and few correlated positively with pH. Key Words: Microbial diversity — PhyloChip G3 — Acidophilic — Thermophilic — Hot springs — Iceland. Astrobiology 14, 229–240.