Apparatus and method for monitoring a vapor deposition installation in which a gas mixture can undergo gas phase nucleation (GPN) and/or chemically attack the product device, under process conditions supportive of such behavior. The apparatus includes a radiation source arranged to transmit source radiation through a sample of the gas mixture, and a thermopile detector assembly arranged to receive output radiation resulting from interaction of the source radiation with the gas mixture sample, and to responsively generate an output indicative of onset of the gas phase nucleation and/or chemical attack when such onset occurs. Such monitoring apparatus and methodology is useful in tungsten CVD processing to achieve high rate tungsten film growth without GPN or chemical attack.