--- a/Xspec/src/XSStat/Cstat.h	2021-02-19 10:47:02.000000000 -0600
+++ b/Xspec/src/XSStat/Cstat.h	2022-02-16 10:37:53.682939519 -0600
@@ -134,10 +134,13 @@
       return;
     }
   } else {
-    Real bTime (background->data()->exposureTime());
+    // tb includes the background exposure time multiplied by the ratios
+    // between the source and background BACKSCALE and AREASCALE.
     RealArray tb (background->data()->backgroundScale()[IN]);
     tb /= sp->backgroundScale()[IN];
-    tb *= bTime;
+    tb *= background->data()->areaScale()[IN];
+    tb /= sp->areaScale()[IN];
+    tb *= background->data()->exposureTime();
     RealArray B (tb * background->spectrum()[IN]);
     RealArray Berr (tb * background->variance()[IN]);
 
@@ -228,10 +231,13 @@
     T::specificResetCalc(S, ts, model, number(), modifier(), diff1, diff2);
   } else {
     // Variable definitions follow Appendix B of manual.
-    Real bTime (background->data()->exposureTime());
+    // tb includes the background exposure time multiplied by the ratios
+    // between the source and background BACKSCALE and AREASCALE.
     RealArray tb (background->data()->backgroundScale()[IN]);
     tb /= sp->backgroundScale()[IN];
-    tb *= bTime;
+    tb *= background->data()->areaScale()[IN];
+    tb /= sp->areaScale()[IN];
+    tb *= background->data()->exposureTime();
     RealArray B (tb * background->spectrum()[IN]);
     RealArray Berr (tb * background->variance()[IN]);