A conventional vertical powder diffractometer has been adapted to allow the collection of high-resolution, single-wavelength diffraction data using Co, Cu or Mo radiation. The major modifications are (i) incorporation of an incident beam focusing monochromator attached to the tube shield, (ii) a variable tilt angle of the tube shield to provide a horizontal beam path through the diffractometer (for ease of alignment), (iii) mounting of the entire diffractometer on a single, very stable base-plate, with micrometer-controlled adjustment of the orientation, (iv) inclusion of a knife-edge, micrometer-controlled focusing slit, and (v) use of a range of Soller slits with acceptance angles down to 1.5° 2φ. The performance of the instrument compares favourably with conventional non-monochromated diffractometer data collected from SRM660 LaB6 and monoclinic ZrO2. In particular, the peaks are more symmetric and have narrower widths, and the peak-to-background ratio is much higher, leading to much superior resolution and profile shapes for structure solution and Rietveld refinement.