Many evolved peraluminous granite plutons contain either andalusite (Al2SiO5) or topaz (Al2SiO4(OH,F)2), but some plutons contain both of these A/NK [mol. Al2O3/(Na2O+K2O)] = ∞ phases. We present the results of experiments conducted to locate the andalusite-topaz boundary in water-saturated peraluminous haplogranite melts in T-P-X space, where T = 700-650°C, P = 200 MPa, and X (A/NK = 1.1, 1.2, 1.3, 1.4, and F = 0, 1, 2, 4 wt.%).The starting materials are synthetic K-Na-Al-Si gels, with added H2O and AgF, with seeds of natural andalusite and topaz, and with run times of 5-7 days. Phase identification is by electron microprobe. All experimental run products contained quartz. For all values of bulk A/NK, F concentrations ≤1 wt.% favour andalusite stability, whereas F concentrations ≥1 wt.% favour topaz stability. For bulk A/NK ≤1.2 and intermediate F concentrations, no A/NK = ∞ phase is present; for bulk A/NK ≥1.3, quench or metastable mullite formed. Topaz reaction rims on naturally occurring andalusite, frequency of naturally occurring assemblages, general chemical considerations, and experimental evidence all suggest that some topaz is the product of a peritectic reaction between an early primary magmatic andalusite and a late F-enriched melt. The appearance of such topaz is an important mineralogical expression of F activity in the melt, and may be an indicator for high field-strength Sn-W-U-Mo polymetallic mineralization.