A thermodynamic model has been used to quantify the effects of transport and deposition on the gold and silver contents, Ag/Au ratios and ΣCO 2/ΣH 2S ratios in boiling hydrothermal solutions. Boiling is singled out because the majority of the precious-metal vein deposits studied exhibit some evidence of two-phase behavior. Several hundred model calculations were carried out for the following conditions: T = 150–300°C, Cl − = 0.1 and 1.0 m, pH = 4–6, ΣH 2S = 0.1–10 −5 m, and ΣH 2S/ΣSO 4 = 10−10 9. The transport calculations indicate that low Ag/Au ratios (<1) in solution result from conditions dominated by low temperature (</250°C), low Cl − (0.1 m), moderate to high pH (⩾5) and ΣH 2S (⩾10 −3 m), and high ΣH 2S/ΣSO 4 ratios (>10 5). At these conditions, argentite is the most stable silver phase and gold is transported dominantly as Au(HS) 2 −. The AuCl 2 − complex dominates at high temperature (⩾250°C), high Cl − (1.0 m), low ΣH 2S (<10 −3 m), low to moderate pH (⩽5), and low ΣH 2S/ΣSO 4 ratios (<10 5). These conditions favor high Ag/Au ratios in solution (> 1) and generally native silver as the most stable silver phase. The overall frequency distribution of the Ag/Au ratios in solution exhibits many similarities with patterns given for natural epithermal vein deposits. Collectively, these distributions display a pronounced mode between Ag/Au values of 10 and 100. The depth at which boiling commences becomes shallower with either decreasing temperature and/or CO 2 concentration for a constant salinity. Solutions with the highest initial gas concentrations will tend to deposit gold and silver over the greatest vertical distances, and produce deposits with lower ore grades. The model also predicts the presence of more intense zones of gold and silver enrichment near the top of the boiling interval which may be analogous to the very high-grade bonanza ores observed in many precious metal vein deposits. However, the low magnitudes of these enrichments during a single boiling event suggest that many volumes of a boiling fluid passing through a given volume of rock are required in order for ore grades in the mineralized boiling zone to reach levels typical of most epithermal deposits. During the boiling process, the Ag/au ratios remain relatively constant except near the top of the boiling interval where abrupt changes are observed. Significant increases in Ag/Au ratios are associated with systems that have moderate to high temperatures, high Cl −, low ΣH 2S, low to moderate pH, and high ΣCO 2/ΣH 2S ratios. Conversely, a drop in the Ag/Au ratios near the top of the boiling zone accompanies systems with low temperature, low Cl −, moderate to high pH, high ΣH 2S and low ΣCO 2/ΣH 2S ratios. The frequency distribution of the boiling Ag/Au ratios is very similar to both the transport histogram and the histograms for natural vein deposits. The Ag/Au mode of 10 to 100 is preserved, and the overall range of values is basically the same (10 −3 to 10 3).