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This letter presents an empirical mixing law for forest fire ash over X-Band (8-12 GHz). Six different species of Australian flora were tested. These include eucalypt, bracken fern, she oak, wattle, cypress, and pine. The investigation highlighted the discrepancies of mixing laws based on a spherical mixing phase to those measured. By optimizing the dimensionless parameter (v) of the unified mixing law formula, a good match was achieved. An average value of v = 10 was achieved over all six samples. The effective solid complex permittivities “εi ” for all the samples were determined. These were eucalypt (6.30 + 0.06 j), bracken fern (4.85 + 0.51 j), she oak (10.05 + 1.76 j), wattle (11.44 + 0.1.71 j), cypress (8.68 + 0.85 j), and pine (16.99 + 3.51 j).