Mercury speciation analysis in produced water (PW) was achieved by optical multipath cold vapor atomic absorption spectrometry. One sample aliquot was treated with SnCl2, producing Hg0 that was carried out to the optical detection cell to be determined. Another aliquot was exposed to UV, promoting oxidation of CH3Hg+ to Hg2+, and then determined after chemical reduction with SnCl2. The CH3Hg+ content was the difference of the results achieved for both aliquots. Matrix effects imposed by oil and salt were studied and although intensity of the mercury time profile was affected, quantification was successfully achieved by using matrix-matched standards containing adjusted NaCl and total oil content. Both conductivity (to estimate salt content) and turbidimetry (to estimate oil content) measurements were made for the PW samples in order to adjust sample to the matrix matched standards. Analyte addition was also used and limit of quantification was 12 ng L−1 with satisfactory recoveries for both Hg2+ and CH3Hg+.