Thorium has only one semi-stable isotope: 232Th, but there are many others that are not naturally produced.
| Isotope | Natural Abundance (%) | Half-life |
| 209Th | 0 | 7 ms |
| 210Th | 0 | 17 ms |
| 211Th | 0 | 48 ms |
| 212Th | 0 | 36 ms |
| 213Th | 0 | 140 ms |
| 214Th | 0 | 100 ms |
| 215Th | 0 | 1.2 s |
| 216Th | 0 | 26.8 ms |
| 217Th | 0 | 240 µs |
| 218Th | 0 | 109 ns |
| 219Th | 0 | 1.05 µs |
| 220Th | 0 | 9.7 µs |
| 221Th | 0 | 1.73 ms |
| 222Th | 0 | 2.237 ms |
| 223Th | 0 | 0.60 s |
| 224Th | 0 | 1.05 s |
| 225Th | 0 | 8.72 min |
| 226Th | 0 | 30.57 min |
| 227Th | 0 | 18.68 d |
| 228Th | 0 | 1.9116 y |
| 229Th | 0 | 7340 y |
| 230Th | 0 | 75,380 y |
| 231Th | 0 | 25.52 h |
| 232Th | 100 | 1.405 x 1010 y |
| 233Th | 0 | 21.83 min |
| 234Th | 0 | 24.10 d |
| 235Th | 0 | 7.2 min |
| 236Th | 0 | 37.5 min |
| 237Th | 0 | 4.8 min |
| 238Th | 0 | 9.4 min |