Second quarter sales totaled $1.03 billion, a 4% increase from the second quarter of 2011. Adjusted for currency and acquisitions, sales were up 3.4%. Adjusted net income per share for the quarter increased to $0.84, a 31% increase from $0.64 in 2011. Our press release includes details, showing the reconciliation of our reported to our adjusted results.
As I comment on our gross margin and operating expense performance, note that restructuring is excluded in both years and acquisition related charges are excluded in the 2011 period.
For the second quarter, our gross margin was 34.6%, up 230 basis points from 2011. Our margins benefitted from carryover pricing actions, which help to offset raw material cost increases. Margins also benefited from productivity improvements, our prior restructuring actions and strong incremental margins on new business. In the quarter we experienced raw material increases sequentially and year-over-year, and we expect mid single-digit increases in the back half of the year.
As a rate to revenue, operating expenses were 21.5%, which were flat versus the second quarter of 2011. Quarter-over-quarter operating expense dollars increased $8.9 million or 4%. The reported tax rate for the second quarter was 31% down from a rate of 33% in the second quarter of last year. The lower rate was due to the geographical mix of earnings, and our full year tax rate guidance remains at 30% to 31%.Average diluted shares outstanding were $95.1 million, down $2.4 million from last year. In the quarter, we continue to return cash to shareholders, repurchasing 2 million shares for approximately $96 million and have 5 million shares remaining under our current authorization. We estimate average diluted shares outstanding for the third quarter to be approximately 95 million. Recapping our sales performance, our core growth was 3.4%, primarily driven by mid-single-digit price increases, which offset a 0.2% decline in volume. The currency impact was neutral and acquisitions added another 0.6% for total growth of 4% in the quarter. Let me provide some additional context on our sales, sequentially our volume trend improved from the first quarter to the second quarter, and we expect this trend to continue in the back half of the year.