Analytical applications of 5-methylfuran-2-carbaxaldehyde thiosemicarbazone (5-MFAT) and 3-methylthiophene-2-carbaxaldehyde thiosemicarbazone (3-MTAT) are reported for the first time. The reagents have been synthesized and characterized using IR, and NMR spectral data. 5-MFAT and 3-MTAT have been used for the spectrophotometric determination of mercury(II). The reagents react with mercury(II) in basic medium (pH 8.0, ammonium chloride and ammonium buffer) to form yellow colored complex 1:2 (M: L) complexes. The color reactions are instantaneous and absorbance values remain constant for 12 hours. The molar absorptivity and sandell's sensitivity of 5-MFAT and 3-MTAT methods are found to be 2.5 x 10⁴ and 1.87 x 10⁴ L mol^-1 and cm^-1 and 0.008 and 0.0107 ìg cm^-2 of mercury(II) respectively. The methods were successfully applied to a number of water (potable and polluted), biological and soil samples containing mercury(II). The results of the proposed method are in agreement with flame atomic absorption spectrophotometry. The methods have high accuracy and precision (s = 0.018 and 0.0042 for 4 ìg/ml in 5-MFAT and 3-MTAT methods respectively).