Enteric methane (CH4) emissions from ruminant livestock greatly contribute to global CH4 production, mitigation of which is essential to slow the impact of climate change. Ruminant feed additives offer a cost-effective means of accomplishing this. Previous research showed that minor elevations to the rumen oxidation reduction potential (ORP) using oxygen-releasing compounds can hinder enteric methanogenesis, given that ruminant methanogens are typically only active at ORPs below -300 millivolts.  In-vitro assessment of these compounds, including liquid hydrogen peroxide (H2O2); encapsulated liquid H2O2 for controlled, slow release; and both calcium and magnesium peroxide (CaO2, MgO2), has demonstrated effective CH4 mitigation potential, with consistent CH4 reductions of >50% observed. An encapsulated format of MgO2 and CaO2 could offer feasibility as a CH4 mitigation feed additive solution in both pasture-based and intensive production systems. However, the influence these oxygen-releasing compounds have on the rumen microbiome requires deeper investigation. This study establishes the in-vitro  influence of ORP modulating compounds on rumen microbial communities. Following co-extraction of Nucleic Acids from rumen fluid, amplicon sequencing of the 16S, 28S and ITS genes was performed on all samples (n=64). Results revealed clear microbiome shifts away from common methanogenic pathways, whereby significant reductions in archaeal species richness and diversity was associated with all compounds to varying extents, with no negative impacts on digestibility of feed from MgO2 and encapsulated liquid H2O2. A clearer understanding of the these microbiome impacts will facilitate successful translation of a ruminant feed additive towards large-scale application on farm.