The Desaturate command converts a color image to grayscale values, but leaves the image in the same color mode. For example, it assigns equal red, green, and blue values to each pixel in an RGB image. The lightness value of each pixel does not change.
Keep in mind that the Desaturate command permanently alters the original image information in the background layer. This command has the same effect as setting Saturation to ‑100 in the Hue/Saturation adjustment. For nondestructive editing, use a Hue/Saturation adjustment layer.