The 10.5% increase in fourth quarter adjusted EBITDA and 200 basis point improvement in adjusted EBITDA margin demonstrate the operating leverage in our business, which was achieved on 5.1% comparable quarter growth in revenues. The 16 new markets we entered since the beginning of 2011 have been building units on lease, and 13 were EBITDA positive for 2012. As these locations mature, we believe our operating leverage should continue to improve.
As of December 31, 2012, we have generated free cash flow for 20 consecutive quarters. Free cash flow for the fourth quarter and full year was $25.9 million and $65.1 million, respectively. Net capital expenditures were $6.7 million in the fourth quarter and $25.8 million for the full year, the majority of which are attributable to lease fleet expansion in our U.K. operations where business conditions remained strong. Debt reduction totaled $26.7 million in the fourth quarter and $53.7 million for the full year, after payment of $10.6 million of financing costs related to our new Credit Agreement and redemption premiums on our 2015 Senior Notes.
As a result of our reduced borrowings and better rates under our new Credit Agreement, interest expense in the fourth quarter of 2012 was reduced by nearly 28% or approximately $3.0 million, compared to the same period of 2011. We expect the August 2, 2012 redemption of our previously issued $150.0 million 2015 Senior Notes to produce in excess of $6.6 million in annualized interest savings based on our current Credit Agreement borrowing rate and debt level. We continue to have a great deal of financial flexibility with $449.2 million available under our Credit Agreement at December 31, 2012.
Throughout the year, we served a large, diverse base of over 83,000 customers, up from 80,000 in 2011, with the growth driven by traction gained by newer locations, recent investments, leadership changes in our National Sales Center and a modest improvement in the economy.