A mechanism is described for adjustment under ultrahigh vacuum of the optical alignment of gratings in a vertically dispersing grazing incidence monochromator. The mechanism is based on two pairs of broad leaf springs and it allows rotation of the grating, of up to one degree, around two mutually perpendicular axes with respect to the photon energy scan axis. Measurements for a grazing incidence monochromator show that to achieve optimum resolution the adjustment of the grating should be better than 0.1°.