The article discusses the composition of studied ore-forming solutions and the P-T conditions of molybdenum mineralization in the Pervomaisky stockwork deposit which is situated within the Dzhidinsky ore field (South-Western Transbaikalia, Russia). New geochronological data of zircons from granites, muscovite, and molybdenite from the ore zones indicates the association of the granite formation and ore deposition processes which occurred 119–128 million years ago. Quartz-molybdenite veins of the Pervomaisky deposit were formed at the temperature of ≥314–186 °C with some boiling periods. Fluid inclusions in these veins have total salt concentration of 6.3–12.7 wt. % NaCl equivalent (eq. NaCl). The salt solution is composed of chlorides of Na, Ca, K, and Fe. The gas phase contains CO2, CH4, and N2. A series of elements were determined in fluid inclusions by laser ablation (LA)-ICP-MS: Li, Be, B, F, Na, Mg, Al, Cl, K, Ca, Mn, Fe, Cu, Zn, Nb, Mo, Ag, Sn, La, Ce, Ta, W, Au, Pb, Th, U. The Mo content reaches 559 ppm (average of 228 ± 190 ppm) in high-grade quartz-molybdenite veinlets, whereas Mo content is up to 212 ppm (average of 25 ± 29 ppm) in the low-grade veinlets. High-grade veinlets were formed by near-neutral solutions with a higher content of Mo, S, and F, while relatively low-grade veinlets were deposited from alkaline solutions. Our results demonstrate the pH of the solutions as one of the key factors for ore deposition.