<p>In subduction zones, carbon-bearing fluids play a pivotal role in deep carbon cycling and global climate change. Given that Earth’s interior is dominated by silicates, the existence of aqueous carbon-silicon (C-Si) species is crucial for assessing the magnitude of return flux of carbon from subducting slabs to mantle wedges and ultimately Earth’s surface, yet it remains controversial. To resolve this controversy, we undertake a comprehensive in situ investigation of high pressure–temperature fluids employing optical cells and Raman spectroscopy under conditions comparable to those of previous studies. Under oxidizing conditions near the C-CO₂ buffer, our findings show no evidence for C-Si species. Notably, under reducing conditions, a shoulder peak near the dominant CH₄ peak, previously interpreted as evidence for C-Si speciation, emerges in high-temperature CH₄-bearing fluids without silicates and even in pure CH₄, suggesting it simply comes from CH₄ hot bands. Therefore, C-Si species are unlikely in subduction-zone fluids.</p><p></p>