Secondary electron (SE) emission is a phenomenon important for technical applications as well as fundamental studies, but yet still not understood well enough today. We employ coincidence spectroscopy to study the SE emission mechanics and show that the coincidence spectra can be used to determine the electron affinity χ of insulating surfaces. This is essential for quantitative understanding of the secondary electron yield since it determines the width of the escape cone. Comparison with Monte Carlo calculations supports our interpretation. The influence of the surface barrier, the energy loss of the primary electron, the number of inelastic scattering events and their depths are investigated in some detail. The elastic and inelastic scattering cross sections σ and MFPs λ for the primary electron are well known. We determine the electron escape probability in dependence of the creation depth and the electron affinity with coincidence measurements and Monte Carlo simulations of Polymethylmethacrylate(PMMA). The coincidence measurement is built up as follows: An electron gun with very low output current I≈2.6 pA and acceleration voltage U= 173 V, i. e. E= 173 eV is directed towards a PMMA sample, where secondary electrons are produced and measured in coincidence with the back scattered primary electrons using a time of flight and an electrostatic analyser. The current is chosen low such that the ratio of true to false coincidences is optimised. Evaluation of the double differential coincidence spectrum provides a value for the electron affinity by means of two different types of analysis, which are in good mutual agreement.