Fourth quarter 2012 capex of $102 million related primarily to the deployment of Clearwire's LTE network as well as ongoing maintenance of our mobile WiMAX network, and increased $68 million and $79 million, respectively, as compared to $34 million in third quarter 2012 and $23 million of capex in fourth quarter 2011. Both the year over year and sequential increases in capex are primarily related to increased network investments associated with our ongoing LTE deployment efforts.
At the end of fourth quarter 2012, Clearwire operated networks in the U.S. covering areas where approximately 137 million people reside, including approximately 135 million people in markets where we provide 4G services, slightly higher than the prior year period.
Results of Operations Cost of goods and services and network costs (COGS) in fourth quarter 2012 decreased 29% to $208.3 million compared to $294.0 million in fourth quarter 2011. These amounts include non-cash charges for network equipment reserves and other write-downs of $2.3 million and $6.4 million in fourth quarters 2012 and 2011, respectively, and other non-cash network-related expenses of $22.9 million and $115.4 million in fourth quarters 2012 and 2011, respectively. The year over year decrease in other non-cash network related expenses is primarily due to a higher provision for unused tower-related leases and other network agreements in fourth quarter 2011. Excluding non-cash expenses, COGS increased 6% year over year primarily due to higher tower- and network-related expenses in conjunction with our ongoing LTE build, as well as an increase in customer premise equipment sales associated with our no contract retail model, which required customers to purchase rather than lease devices beginning in 2012.
Selling, general and administrative (SG&A) expense in fourth quarter 2012 increased 8% to $138.5 million compared to $128.5 million in fourth quarter 2011. The increase is primarily attributable to professional fees related to the proposed merger with Sprint and employee-related expenses including stock compensation.