diff --git a/src/phreeqcinwt/phreeqc_wt_api.py b/src/phreeqcinwt/phreeqc_wt_api.py
index c636d46..967a75e 100644
--- a/src/phreeqcinwt/phreeqc_wt_api.py
+++ b/src/phreeqcinwt/phreeqc_wt_api.py
@@ -794,6 +794,11 @@ def solution_modify(
             command += "   -temp {}\n".format(temperature)
         if pressure is not None:
             command += "   -pressure {}\n".format(pressure)
+        # H and O must use dedicated keywords, not -totals
+        if "H" in totals:
+            command += "   -total_h {:.15e}\n".format(totals.pop("H"))
+        if "O" in totals:
+            command += "   -total_o {:.15e}\n".format(totals.pop("O"))
         if totals:
             command += "   -totals\n"
             for phreeqc_name, mols in totals.items():
diff --git a/src/phreeqcinwt/tests/test_solution_build.py b/src/phreeqcinwt/tests/test_solution_build.py
index e905c2b..65a5d8a 100644
--- a/src/phreeqcinwt/tests/test_solution_build.py
+++ b/src/phreeqcinwt/tests/test_solution_build.py
@@ -536,6 +536,41 @@ def test_large_combined_step(self, modify_base):
             cl_before + cl_delta
         ), f"{db}: large combined Cl mismatch"
 
+    # -- H and O via total_h / total_o -------------------------------------
+
+    def test_absolute_h_and_o(self, modify_base):
+        """Modifying H and O should use -total_h / -total_o internally."""
+        wt, initial_state, db = modify_base
+        elems = initial_state["composition"]["elements"]
+        h_before = elems["H"]["mols"]
+        o_before = elems["O"]["mols"]
+        # Small 2 % bump to both — no charge issue for H2O bulk totals
+        target_h = h_before * 1.02
+        target_o = o_before * 1.02
+        result = wt.solution_modify(absolute={"H": target_h, "O": target_o})
+        assert result["composition"]["elements"]["H"]["mols"] == _approx(
+            target_h
+        ), f"{db}: absolute H mols mismatch"
+        assert result["composition"]["elements"]["O"]["mols"] == _approx(
+            target_o
+        ), f"{db}: absolute O mols mismatch"
+
+    def test_relative_h_and_o(self, modify_base):
+        """Relative H and O changes should apply via -total_h / -total_o."""
+        wt, initial_state, db = modify_base
+        elems = initial_state["composition"]["elements"]
+        h_before = elems["H"]["mols"]
+        o_before = elems["O"]["mols"]
+        h_delta = h_before * 0.05
+        o_delta = o_before * 0.05
+        result = wt.solution_modify(relative={"H": h_delta, "O": o_delta})
+        assert result["composition"]["elements"]["H"]["mols"] == _approx(
+            h_before + h_delta
+        ), f"{db}: relative H mols mismatch"
+        assert result["composition"]["elements"]["O"]["mols"] == _approx(
+            o_before + o_delta
+        ), f"{db}: relative O mols mismatch"
+
     # -- error handling (single-database, no fixture) -----------------------
 
     def test_relative_without_state_raises(self):