Absolute dating of diagenesis is a challenging, yet significant problem. Reconstructing the depositional and diagenetic history of a late Pleistocene coralligéne-type red algal reef and overlying maerl (red algal sand) from the Plimiri coastal cliff, SE island of Rhodes, Greece, is enabled using a combination of microstructure, stable-isotope analyses, and electron spin resonance of large bivalve shells partially altered by diagenesis. These shells, consisting of both original shell aragonite and polymorphic transformed rims (now consisting of neomorphic spar), provide the opportunity to date the primary age when the shells were precipitated, as well as the age of their polymorphic transformation. The contamination of samples presented a significant problem for interpreting ages based on data from electron spin resonance (ESR) alone. However, sample contamination was evaluated with quantitative XRD, providing the opportunity to apply corrections, which increased reliability of the ages. The carbon and oxygen stable-isotope values and the neomorphic spar microtexture suggested that the polymorphic transformation at Plimiri took place in a meteoric environment with a high and increasing fluid:rock ratio. The electron-spin-resonance dates combined with the vertical sedimentary evolution allowed to assign the formation of the red algal reef to the Marine Isotope Stages 6 to 5e transgressive event (ca 135 to 120 ka), and to identify meteoric diagenetic phases during possibly the Marine Isotope Stage 5d and the upper Marine Isotope Stage 5c to lower 3. The use of electron-spin-resonance dating of neomorphic spar, and age correction for sample contamination is presented herein for the first time and the results suggest that it might provide a useful tool to date late Pleistocene diagenesis.