We present a new and transformative composition for Venus' cloud aerosols using previously uncharacterized data acquired by the Pioneer Venus Large Probe, which descended through Venus' atmosphere in 1978. This aerosol composition was extracted by re‐analyzing data acquired by the Large Probe Neutral Mass Spectrometer (LNMS) and re‐evaluating results from the Large Probe Gas Chromatograph (LGC). In our approach, the altitude‐based data from the LNMS and LGC were re‐interpreted as the thermal and evolved gas analysis of aerosol composition. During the descent, the LNMS and LGC inadvertently collected cloud aerosols and subsequently measured the release of gases and compounds as the captured aerosols thermally decomposed across the hot sub‐cloud atmosphere. Evolved compounds that were released into the LNMS included SO2, H2O, SO3, O2, and likely Fe2O3 and MgSO4. Evolved gases that were released into the LGC included SO2, H2O, and O2, which likely formed during the programmed LGC heating steps. These results are consistent with the thermal decomposition of aerosols containing comparable masses of ferric sulfate and sulfuric acid (∼20 wt%, each) and sufficiently high H2O abundances (∼60 wt%) to yield hydrated iron sulfates, hydrated magnesium sulfate, and other hydrates. We suggest that iron and magnesium could originate from cosmic sources. Our comparisons indicate that aerosol H2O was also measured by direct analyses on the Venera and Vega probes. Hence, this work reveals reservoirs of water and possible altered cosmic materials in the aerosols, which presents new considerations for cloud chemistry and cloud habitability models.