Chemical vapor deposition (CVD) is the broadly used technique in the synthesis of 2D materials for various electronic and optoelectronic devices. In this study, we aim to understand the precise phase transition of molybdenum trioxide (MoO₃), the most commonly used transition metal precursor, under the influence of sulfur in a CVD process that ultimately leads to the growth of mono-to few-layer molybdenum disulfide (MoS₂) with varying morphologies. For this, we employed a space-confined growth method and carried out a methodical spectroscopic analysis to understand the step-by-step reduction of MoO₃ under reaction with sulfur vapors and its subsequent transformation to MoS₂ on a SiO₂ substrate. The sulfur-promoted reduction of MoO₃ forming molybdenum dioxide (MoO₂), followed by molybdenum oxysulfide (MoOS₂) to MoOS₂/MoS₂ hybrid crystals, and finally monolayer (few layer) triangle (hexagonal) MoS₂ crystals, is discussed. The results show that the nucleation presumably initiated by oxy-chalcogenide micro-particles that serve as heterogeneous monolayer nucleation sites. We validate our findings with optical and scanning electron microscope images, micro-Raman, photoluminescence (PL), and x-ray photoelectron spectroscopy (XPS) studies. Our findings can contribute to a better understanding of the growth processes of sequential sulfurization reactions.