Process Products segment revenue decreased $4.2 million or 12.8 percent to $28.5 million. Customer-driven delays in the required completion dates on certain orders in our backlog have resulted in lower revenue this quarter than in the comparable period in the prior year. Segment operating income decreased $0.8 million or 14.8 percent to $4.7 million on lower revenue, partially offset by an improvement in relative gross margin. The segment gross margin was positively impacted by product and geographical mix in the quarter.
The Environmental Systems segment revenue decreased $2.1 million or 41.2 percent to $3.0 million. The decrease in revenue is attributed to the dampened demand for SCR equipment in the United States resulting from delays in the release of finalized environmental regulations. Operating income decreased $0.4 million to $0.5 million for the quarter on the lower revenue.
Fiscal Year To-Date 2013 Compared to 2012Revenue for the six month period ended December 29, 2012 decreased $2.4 million or 3.6 percent to $64.4 million. Revenue growth in the Asia-Pacific region and the benefit of the acquisition of Burgess-Manning GmbH in November 2011 were more than offset by declines in domestic revenue in both our Process Products and Environmental Systems segments. Net income attributable to PMFG, Inc. common stockholders was $0.2 million compared to a net loss of $1.1 million in the prior year. Net income attributable to PMFG, Inc. common stockholders on a non-GAAP basis of $0.3 million or $0.02 per diluted share in fiscal 2013 held constant with the prior year. Net Bookings and Backlog Net bookings totaled $27.7 million and $52.4 million during the three and six month periods ended December 29, 2012, respectively. As anticipated, net bookings in the quarter and year to date decreased significantly from the prior year as we overlapped significant international bookings received in the second quarter of fiscal 2012. The backlog at December 29, 2012 was $87.9 million of which approximately 14% remains on customer hold. Although the customer has communicated that they expect the capital project to move forward, the timing and impact on our project scope is not yet known.