Analysis of mantle discontinuity structure using converted P to S (Pds) phases beneath Colorado from the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) Rocky Mountain Front (RMF) experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations. A stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km, respectively. Imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the Pds conversion points and then stacking the receiver functions in each bin to form spatial images, analogous to common depth point stacking. Corrections for lateral velocity heterogeneity are calculated using the local S wave tomographic model of Lee and Grand <1996> and a constant ∂lnVs/∂lnVp scaling of 1.3. This scaling value is determined from the relative scaling between teleseismic P and S wave travel time residuals measured from the Rocky Mountain Front deployment. Mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography, 30 km of 660 km discontinuity topography, and up to 40 km of transition zone thickness variation. Features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western Colorado and a gradual 10 km dip of the 410 km discontinuity beneath the High Plains. The thickening of the transition zone beneath southwest Colorado is consistent with the presence of the subducted Farallon slab in this region as imaged by Van der Lee and Nolet <1997>. In general, our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies. ¿ 1998 American Geophysical Union