diff --git a/reactor.html b/reactor.html
index 3ccff6b..5990f85 100644
--- a/reactor.html
+++ b/reactor.html
@@ -35,7 +35,7 @@
             X_S_init: { value: 75., required: true },
             X_H_init: { value: 30., required: true },
             X_STO_init: { value: 0., required: true },
-            X_A_init: { value: 0.001, required: true },
+            X_A_init: { value: 200, required: true },
             X_TS_init: { value: 125.0009, required: true },
 
             timeStep: { value: 1, required: true },
diff --git a/test/basic/timestep-units.basic.test.js b/test/basic/timestep-units.basic.test.js
new file mode 100644
index 0000000..09feadf
--- /dev/null
+++ b/test/basic/timestep-units.basic.test.js
@@ -0,0 +1,44 @@
+'use strict';
+
+// Locks in the contract that `config.timeStep` is interpreted as SECONDS by
+// the reactor kinetics engine. Before 2026-05-19 the schema labelled the field
+// `unit: "h"` while reactor.html labelled it `[s]` and baseEngine divided by
+// 86400 (seconds-per-day) to convert to internal days. A 0.001 schema default
+// — read as hours — would have produced a 3.6 s step; read as seconds it is a
+// 1 ms step. The fix aligned the schema to seconds. This test prevents the
+// drift from reappearing.
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const { Reactor_CSTR } = require('../../src/specificClass');
+const { makeReactorConfig } = require('../helpers/factories');
+
+const SECONDS_PER_DAY = 24 * 60 * 60;
+
+function makeEngine(timeStepSeconds) {
+  return new Reactor_CSTR(makeReactorConfig({ reactor_type: 'CSTR', n_inlets: 1, timeStep: timeStepSeconds }));
+}
+
+test('engine stores timeStep in days, treating input as seconds', () => {
+  const eng = makeEngine(1);
+  assert.ok(Math.abs(eng.timeStep - 1 / SECONDS_PER_DAY) < 1e-15,
+    `engine.timeStep should be 1/86400 days for a 1-second config; got ${eng.timeStep}`);
+});
+
+test('engine timeStep scales linearly with config.timeStep (seconds in)', () => {
+  const a = makeEngine(1);
+  const b = makeEngine(10);
+  assert.ok(Math.abs(b.timeStep - 10 * a.timeStep) < 1e-15,
+    'engine.timeStep must scale linearly with config.timeStep; broke the seconds→days conversion');
+});
+
+test('schema default for timeStep matches the seconds convention', () => {
+  const path = require('node:path');
+  const gfRoot = path.dirname(require.resolve('generalFunctions'));
+  const schema = require(path.join(gfRoot, 'src/configs/reactor.json'));
+  assert.equal(schema.reactor.timeStep.rules.unit, 's',
+    'schema timeStep.unit must be "s" — engine treats input as seconds');
+  assert.equal(schema.reactor.timeStep.default, 1,
+    'schema timeStep.default must be 1 (1 second), matching reactor.html');
+});
diff --git a/test/helpers/factories.js b/test/helpers/factories.js
index 892670e..3ae9085 100644
--- a/test/helpers/factories.js
+++ b/test/helpers/factories.js
@@ -21,7 +21,7 @@ function makeUiConfig(overrides = {}) {
     X_S_init: 75,
     X_H_init: 30,
     X_STO_init: 0,
-    X_A_init: 0.001,
+    X_A_init: 200,
     X_TS_init: 125,
     timeStep: 1,
     enableLog: false,