diff --git a/solidity/saving_gas.md b/solidity/saving_gas.md
index 8c682d9..12db511 100644
--- a/solidity/saving_gas.md
+++ b/solidity/saving_gas.md
@@ -115,6 +115,18 @@ uint256 world; //good, cheap
 
 <br>
 
+---
+
+<br>
+
+## negative gas costs
+
+- Deleting a contract (SELFDESTRUCT) is worth a refund of 24,000 gas.
+- Changing a storage address from a nonzero value to zero (SSTORE[x] = 0) is worth a refund of 15,000 gas.
+
+<br>
+
+
 ---
 
 ## resources and tools