This study aimed to evaluate the potential of image ratioing techniques to detect and map the extent and particle composition of sand dunes in the Erg of Gavkhouni in Isfahan province. The 6/4 ratio index and also a new index called SaDI which was calculated using Landsat band 3, 6 and 7 were applied to a georeferenced Landsat TM image. To evaluate the performance of ratio techniques, the extracted images were correlated with the percentage of different particles collected in 33 sampling sites. Results showed that both ratioing techniques were able to detect and map the spatial distribution of sand dunes and there was more than 70% agreement between image indices and field data. The results also indicated that the performance of SaDI was 50% better in mapping carbonate particles and this confirms that this index is more appropriate than 6/4 index for mapping sand dunes dominated by carbonate particles.