This dataset originates from a microcosm experiment conducted in the Fram Strait to monitor microbial and chemical changes during sea-ice melt. It includes the molecular composition of dissolved organic matter (DOM) analyzed using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The code used to analyze the data can be found here: https://github.com/matthiaswietz/Arctic_IceSeeding. DOM composition was measured on a SolariX XR FT-ICR-MS (Bruker Daltonik GmbH, Bremen, Germany), featuring a 15 Tesla superconducting magnet and equipped with a Bruker Apollo II electrospray ionization (ESI) source. Molecular formula attribution included the elements C (1–100), ¹³C (0–2), H (1–200), O (0–50), ¹⁸O (0–1), N (0–4), ¹⁵N (0–1), S (0–2), ³⁴S (0–1), and P (0–1), with a matching tolerance of 0.5 ppm. When multiple molecular formulae matched a given mass, preference was given to those with isotope verification. Isotope verification required the presence of at least one isotopologue with an intensity ratio deviation within ±1000 permille of natural abundance for Δ¹³C, Δ¹⁸O, Δ¹⁵N, and Δ³⁴S. If multiple isotope-verified formulae remained, the formula with the largest homologous series network, defined by CH₂ and O patterns, was selected.