The EPA is staying the effectiveness of two subcategories of the National Emission Standards for Hazardous Air Pollutants (NESHAP) for stationary combustion turbines: Lean premix gas-fired turbines and diffusion flame gas-fired turbines. Pending the outcome of EPA's proposal to delete these subcategories from the source category list (68 FR 18338, April 7, 2004), EPA is staying the effectiveness of the emissions and operating limitations in the stationary combustion turbines NESHAP for new sources in the lean premix gas-fired turbines and diffusion flame gas-fired turbines subcategories. This action is necessary to avoid wasteful and unwarranted expenditures on installation of emission controls which will not be required if the subcategories are delisted.