The operating margin expansion was driven by a balanced contribution of gross margin expansion and operating cost leverage. Gross margins were at or above 20% in all four quarters of 2011 highlighting a continued effectiveness of our sales and marketing programs and a positive impact of our margin improvement initiative.
These results achieve the targets we outlined in our Investor Day in August of last year. Our investments are clearly paying off. Effective execution of our growth strategy continues and we are pleased with the positive momentum and improved profitability of our business in 2011.
I am very proud of the extra effort and results delivered by our One WESCO team of associates around the world and I would like to thank them for their dedication and commitment in serving our customers.
On January 4th, we completed the acquisition of RS Electronics, our fifth acquisition over the last 18 months. These five acquired companies had annualized sales of approximately $450 million as of their respective closing dates. With liquidity increasing to over $500 million and leverage dropping 2.3 to start the year, we have the capacity and financial flexibility to continue to fund our strategy of above-market organic growth plus accretive acquisitions. Our acquisition pipeline remains robust and we see excellent opportunity for acquisitions to further strengthen our portfolio.
In summary, we enter 2012 with a stronger and more diverse business with respect to customers in end markets, products and suppliers and geography. The strength, diversity and operating leverage of our enterprise position us well in our global markets. We are continuing to invest in our eight growth engines and our six margin and productivity initiatives.
Our long term outlook remains unchanged. We expect the economy to continue to recover slowly over the next several years. Our 2012 target of 7% to 11% top-line growth, sales growth, including a 2 to 3 percentage point contribution from acquisition and 20% to 25% annual net income growth remain intact.
We are focused on building on the positive momentum across WESCO as we continue to execute our One WESCO growth strategy from 2012, and I'm very confident in our team's ability to produce excellent results again this year.
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