Answering the question of whether life ever existed on Mars is a key goal of both NASA’s and ESA’s imminent Mars rover missions. The obfuscatory effects of oxidising salts, such as perchlorates and sulfates, on organic matter during thermal decomposition analysis techniques are well established. Less well studied are the transformative effects of iron oxides and (oxy)hydroxides, which are present in great abundances in the martian regolith. We examined the products of flash pyrolysis GC-MS, a technique analogous to the thermal techniques employed by past, current and future landed Mars missions, formed when the cyanobacteria Arthrospira platensis was heated in the presence of a variety of Mars-relevant 16iron bearing minerals. We found that iron oxides/(oxy)hydroxides have transformative effects on the pyrolytic products of cyanobacterial biomolecules. Both the abundance and variety of molecular species detected were decreased as iron substrates transformed biomolecules, by both oxidative and reductive processes, into lower fidelity alkanes, aromatic and aryl-bonded hydrocarbons. Despite the loss of fidelity, a suite containing mid-length alkanes and PAHs and/or aryl-bonded molecules in iron-rich samples subjected to pyrolysis, may allude to the transformation of cyanobacterially-derived mid-long chain length fatty acids (particularly unsaturated fatty acids) originally present in the sample. Haematite was found to be the iron-oxide with the lowest transformation potential and so, because this iron oxide has a high affinity forco-deposition of organic matter and preservation over geological timescales, sampling at Mars should target sediments/strata which have undergone a diagenetic history encouraging the dehydration, dehydroxylation and oxidation of more reactive iron-bearing phases to haematite by looking for (mineralogical) evidence of the activity of oxidising, acidic/neutral and either hot or long-lived fluids.