This dataset contains carbon (C), hydrogen (H), nitrogen (N) and sulphur (S) concentrations determined by combustion analysis for a suite of samples collected from deposits associated with the 1877 eruption of Cotopaxi Volcano, Ecuador. Analyses were performed using an Elemental Analyser (IsoCube, Elementar®) operating in CHNS mode to characterise the light-element geochemistry of volcanic materials. The dataset covers samples collected from locations around the flanks of Cotopaxi Volcano, Ecuador. The data are provided in Microsoft Excel (.xlsx) format and comprise a worksheet containing sample information and metadata together with a worksheet containing raw CHNS analytical results and associated standard measurements. This dataset was generated, between June 2025 and April 2026, as part of NERC project NE/W008033/1 The explosivity of volcanic eruptions: Quantifying the critical role of permeability in magmas.