Fluorescent X-rays from the surfaces of airless planetary bodies in the inner solar system have been measured by instruments on several spacecraft. X-ray emission follows photoionisation by incident solar X-rays and charged particles and reveals the elemental composition of the surface. Analyses of X-ray spectra obtained by orbiting spacecraft, use the relative intensities of elemental emission lines (e.g., Ca/Si, Fe/Si) to determine the geochemistry of the target body. Historically, the analysis of X-ray spectra has assumed that surfaces can be considered as homogeneous plane-parallel media. It has been shown, however, that relative line intensities are affected by the physical properties of the target surface (e.g. particle size distribution and packing density of the regolith) and the viewing and illumination geometry of observations. We describe experimental investigations into the effects of regolith properties on the line ratios measured by a nadir pointing (emergence angle 0° ) orbiting instrument, with with solar illumination angles in the range 25-75° from zenith. The planetary regolith analogue used in these experiments was a terrestrial, olivine rich basalt, which has been used by previous authors as an analogue to the lunar maria. The basalt samples were ground to powder and sieved to discriminate particles in the ranges, <75µm, 75-250µm, and 250-500µm. These separate powders were then pressed into solid pellets. The separation of particles with different sizes allows some determination of the effects due to changes in particle size. All measurements were made at pressures of less than 0.5 mbar to prevent absorption of fluorescent X-rays in air. The relative fluorescent line ratios of several major rock forming elements (K, Ca, Ti, Si) were measured. We find that for measurements made in a "nadir" pointing geometry, the measured spectrum becomes increasingly hard as illumination angle increases (i.e. X-ray lines at higher energies are enhanced relative to those at lower energies). Some hardening of spectra is predicted by the fundamental parameters equation (FPE) of X-ray fluorescence, which assumes a smooth flat and homogeneous surface, but we observe that spectral hardening is also a function of grain size. In a nadir illumination geometry, the regolith effect works adverse to the FPE induced softening of the spectrum, in effect rendering the observed relative line intensity ratio almost constant over the measured phase angles. In addition to experimental studies we have simulated the X-ray emission from a regolith using a numerical Monte-Carlo ray-trace model. This model simulates a regolith of spherical particles, with defined physical properties (particle size distribution, packing density etc.). We present a comparison of our latest results with those of previous studies and discuss the importance of our work for present and future missions, including Kaguya and Chandrayaan-1 at the Moon and MESSENGER and BepiColombo at Mercury.