Research has shown that mandibular premolars have the highest rate of orthodontic bracket bond failure. Bond strength is thought to depend on the quality and quantity of enamel etch produced by the acid. Phosphoric acid is the most commonly used acid for etching before bonding. Nitric acid is also commercially available for etching enamel; however, it has not been studied specifically for etching in orthodontic bonding. This ex vivo study compared the enamel etch patterns achieved on the orthodontic bonding area of extracted mandibular premolars treated with 37% wt/wt phosphoric acid and 2.5% wt/wt nitric acid applied for 15, 30, or 60 seconds. The etch patterns were viewed with a scanning electron microscope and assessed with a 5-point etch scale. Statistical analysis showed that increasing the duration of acid application (irrespective of acid type) significantly increased the amount of better quality etch. Phosphoric acid at 37% concentration was more effective at producing a good-quality etch than was nitric acid at 2.5% for all 3 application times. The quantity of good-quality etch produced by phosphoric acid at 37% was time specific, with 15 seconds being significantly less effective than 30 or 60 seconds. However, 60 seconds was not significantly better than 30. Comparing the upper half of the orthodontic bonding area with the lower half showed no statistically significant differences for either acid type or length of application (P >.005). These findings support the use of 37% wt/wt phosphoric acid and indicate an optimum application time of 30 seconds.