Using Excel from Threads

I am trying to access Excel spreadsheets to modify data from a VB.NET
application. If I have an object defined as:

Public Class Test
Private _excel As Microsoft.Office.Interop.Excel.Application
Public Property ExcelApp() As
Microsoft.Office.Interop.Excel.Application
Get
Return _excel
End Get
Set(ByVal Value As Microsoft.Office.Interop.Excel.Application)
_excel = Value
End Set
End Property
Public Sub Process()
Dim oBook As Microsoft.Office.Interop.Excel.Workbook
do somethings here before using excel
oBook = _excel.Workbooks.Open("myfile.xls")
do something with oBook
End Sub
End Class

and access like:
Dim otest As New Test
otest.ExcelApp = New Microsoft.Office.Interop.Excel.Application
otest.Process()

This works fine. But if I try to use a thread like:
Dim otest As New Test
otest.ExcelApp = New Microsoft.Office.Interop.Excel.Application
Dim t As New System.Threading.Thread(AddressOf otest.Process)
t.Start()

the code dies. I'm tracing it through with the debugger. The code
gets to the line:
oBook = _excel.Workbooks.Open("myfile.xls")
and when I press F10 and it never comes back.
Is there an issue with using threads and Excel? I've tested using
just one thread so I don't think it is a deadlock issue.

Thanks for any help,

Steve