diff --git a/src/pair.cpp b/src/pair.cpp
index 432680b324..db9792cdbc 100644
--- a/src/pair.cpp
+++ b/src/pair.cpp
@@ -86,6 +86,7 @@ Pair::Pair(LAMMPS *lmp) : Pointers(lmp)
   manybody_flag = 0;
   offset_flag = 0;
   mix_flag = GEOMETRIC;
+  mixed_flag = 1;
   tail_flag = 0;
   etail = ptail = etail_ij = ptail_ij = 0.0;
   ncoultablebits = 12;
@@ -251,10 +252,12 @@ void Pair::init()
 
   cutforce = 0.0;
   etail = ptail = 0.0;
+  mixed_flag = 1;
   double cut;
 
   for (i = 1; i <= atom->ntypes; i++)
     for (j = i; j <= atom->ntypes; j++) {
+      if ((i != j) && setflag[i][j]) mixed_flag = 0;
       cut = init_one(i,j);
       cutsq[i][j] = cutsq[j][i] = cut*cut;
       cutforce = MAX(cutforce,cut);
diff --git a/src/pair.h b/src/pair.h
index 0664e97462..00f433233e 100644
--- a/src/pair.h
+++ b/src/pair.h
@@ -105,6 +105,7 @@ class Pair : protected Pointers {
   int allocated;                 // 0/1 = whether arrays are allocated
                                  //       public so external driver can check
   int compute_flag;              // 0 if skip compute()
+  int mixed_flag;                // 1 if all itype != jtype coeffs are from mixing
 
   enum{GEOMETRIC,ARITHMETIC,SIXTHPOWER};   // mixing options
 
diff --git a/src/write_data.cpp b/src/write_data.cpp
index 511552cf69..d549d8bf77 100644
--- a/src/write_data.cpp
+++ b/src/write_data.cpp
@@ -303,6 +303,9 @@ void WriteData::force_fields()
 {
   if (force->pair && force->pair->writedata) {
     if (pairflag == II) {
+      if ((comm->me == 0) && (force->pair->mixed_flag == 0))
+        error->warning(FLERR,"Not all mixed pair coeffs generated from mixing. "
+                       "Use write_data with 'pair ij' option to store all pair coeffs.");
       fmt::print(fp,"\nPair Coeffs # {}\n\n", force->pair_style);
       force->pair->write_data(fp);
     } else if (pairflag == IJ) {