A liquid delivery system (10) for vaporization of a liquid pecursor to form precursor vapor for transport to a deposition zone (36). The system includes a vaporization chamber (12), a porous vaporization element (60) positioned in the vaporization chamber (12) to define an upstream portion (20) of the vaporization chamber (12) upstream of the porous vaporization element (60), and a downstream portion (22) of the vaporization chamber (12) downstream of the porous vaporization element (60) therein, a vaporization element heater (59), a liquid precursor supply (26) for introducing liquid precursor to the vaporization element (60) for vaporization thereof, a precursor vapor discharge (16) from the chamber, and means (46, 48, 50, 52, 54, 56 and 58) for diverting at least a part of the fluid in the upstream portion of the vaporization chamber (12) past the porous vaporization element (60), to stabilize fluid pressure in the upstream portion of the vaporization chamber (12) under conditions tending to perturb such fluid pressure.