diff --git a/src/post_process/m_data_input.f90 b/src/post_process/m_data_input.f90
index a3e3f22886..6eee4e3f8b 100644
--- a/src/post_process/m_data_input.f90
+++ b/src/post_process/m_data_input.f90
@@ -312,6 +312,10 @@ impure subroutine s_read_serial_data_files(t_step)
                       STATUS='old', ACTION='read')
                 read (1) q_cons_vf(i)%sf(0:m, 0:n, 0:p)
                 close (1)
+            else if (bubbles_lagrange .and. i == beta_idx) then
+                ! beta (Lagrangian void fraction) is not written by pre_process
+                ! for t_step_start; initialize to zero.
+                q_cons_vf(i)%sf(0:m, 0:n, 0:p) = 0._wp
             else
                 call s_mpi_abort('File q_cons_vf'//trim(file_num)// &
                                  '.dat is missing in '//trim(t_step_dir)// &
diff --git a/src/simulation/m_data_output.fpp b/src/simulation/m_data_output.fpp
index 5051d5e15d..096b16d91d 100644
--- a/src/simulation/m_data_output.fpp
+++ b/src/simulation/m_data_output.fpp
@@ -476,6 +476,19 @@ contains
             write (2) q_cons_vf(i)%sf(0:m, 0:n, 0:p); close (2)
         end do
 
+        ! Lagrangian beta (void fraction) written as q_cons_vf(sys_size+1) to
+        ! match the parallel I/O path and allow post_process to read it.
+        if (bubbles_lagrange) then
+            write (file_path, '(A,I0,A)') trim(t_step_dir)//'/q_cons_vf', &
+                sys_size + 1, '.dat'
+
+            open (2, FILE=trim(file_path), &
+                  FORM='unformatted', &
+                  STATUS='new')
+
+            write (2) beta%sf(0:m, 0:n, 0:p); close (2)
+        end if
+
         if (qbmm .and. .not. polytropic) then
             do i = 1, nb
                 do r = 1, nnode