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Pulsed electron electron double-resonance spectroscopy (PELDOR [1] , also called DEER (double electron electron resonance [2] )) is a unique method to determine distance distributions between two or more paramagnetic centers ranging from 2-10 nm. It is widely employed in structural biology and materials science. [3] Nitroxides, usually covalently attached to specific sites of the macromolecule, are an important application, used mostly as spin labels. [4] Such spin labels, which do not occur in the natural form of the biomolecule, exhibit additional degrees of conformational flexibility, which have to be modeled by molecular dynamics or rotamer libraries. [5] However, their linewidth is sufficiently narrow that good inversion efficiency can be achieved with rectangular microwave pulses. A second category of paramagnetic centers, interesting in their own right, are naturally occurring transition-metal ions and iron-sulfur clusters. Unfortunately, PELDOR experiments on this group are much more demanding, owing to their large EPR linewidth. In most cases, this inhomogeneous EPR linewidth far exceeds the excitation and, hence, inversion bandwidth of rectangular microwave pulses. This leads to a poor inversion efficiency of the pump pulse in the PELDOR experiment, causing low modulation depth and therefore reduced signal quality. Nevertheless PELDOR has been successfully applied to specific cases, such as copper complexes, [6] iron-sulfur centers, [7] gadolinium complexes, [8] and manganese ions. [9] In all of these cases, the insufficient inversion of the EPR line by monochromatic rectangular pulses limits the quality of the data. Polychromatic pulses created by five microwave sources have been used to increase the bandwidth of the pump-pulse excitation on the manganese-tyrosine spin pair in photosystem II. [9b] It has been shown in NMR and EPR spectroscopy that amplitude and phase modulation can increase the excitation bandwidth under the condition of limited peak excitation power. [10] Recently, we implemented a fast microwave pulse shaping unit for amplitude and phase modulation on a commercial pulsed EPR spectrometer and demonstrated significantly increased excitation bandwidth for Fourier transform (FT)-EPR. [10g] Here, we demonstrate such concepts for broadband inversion pulses, which strongly increase the modulation depth and therefore the signal-to-noise ratio of PELDOR time traces. The PELDOR experiments have been carried out on a nitroxide-nitroxide and a cobalt-nitroxide biradical (shown in Figure 2 and Figure 3), both synthesized in our group. The Co 2+ -nitroxide biradical was chosen to demonstrate the effect of improved pump-pulse bandwidth on a paramagnetic center with a broad spectral width.The Co 2+ -nitroxide system was studied as a 0.2 mm solution in toluene with three equivalents of pyridine as axial ligands. The bisnitroxide system was immobilized in oterphenyl at a concentration of 0.2 mm. The PELDOR measurements on the bisnitroxide were carried out at 298 K, whereas the measurements o...