If you split the DLOOKUPs into single calls and print them do they have a value?

Hey Alex,
I've tried that, but no value at all.

Change this portion:

    DLookup("NaamOpdrachtgever", "Bedrijven", "Bedrijf='" & Bedrijf & "'", 0)

to this:

    DLookup("NaamOpdrachtgever", "Bedrijven", "Bedrijf='" & Bedrijf & "'")

Just a slight syntax error.

Hey Kevin,
I've made the change, but still no value in that field.
What else could it be?

If you hardcode the DLOOKUP with a value you know exists does it return anything?

Is Bedrijf a string?

In the Table "Bedrijven" I can hardcode the Field with the value ' 23,50. That works if I open the Form "Bedrijfsgegevens".
"Bedrijf" is a Short Text Field, the name of a company, within the Table Bedrijven. It's not a string.

If you add a button to the form and MsgBox Bedrijf what do you get?

Short Text = String
The reason I asked was people wrap Numbers in quotes which a usual error.

Added the Button. It gives me the Name of the Company. Which is good.

Hi Klaas, I spot another error.  This portion:

   DLookup("FormaatContainer1", "Bedrijven", "Bedrijf='" & Bedrijf & "'" AND "NaamOpdrachtgever=" ... ...

should be:

   DLookup("FormaatContainer1", "Bedrijven", "Bedrijf='" & Bedrijf & "' AND NaamOpdrachtgever="

The keyword "AND" should be inside the string, not outside.

Sorry Kevin, if I change it to Bedrijf & "' AND NaamOpdrachtgever="..... the "' returns automatically back to "'".

Make sure there is no line break within "' AND NaamOpdrachtgever=".  If you accidentally hit Enter after "', Access will auto-correct it to "'".  But "AND" has to be inside that string, that is for certain.  

You may consider rewriting the code in smaller pieces, both to improve readability and help you troubleshoot:

Private Sub PrijsPerReiniging_AfterUpdate()
   Dim c1 As Variant, c2 As Variant
   c1 = DLookup("NaamOpdrachtgever", "Bedrijven", "Bedrijf='" & Bedrijf & "'")
   MsgBox "c1 = " & c1
   c2 = DLookup("FormaatContainer1", "Bedrijven", "Bedrijf='" & Bedrijf & "' AND NaamOpdrachtgever=" & c1)
   MsgBox "c2 = " & c2
   PrijsPerReiniging = DLookup("PrijsPerReiniging", "PrijslijstenOpdrachtgeverT", "FormaatContainer1=" & c2)
End Sub

The above MsgBox statements help you check if the expected values are returned.

Hey Kevin, I still had an issue with the last DLookup.
I fixed it with:
PrijsPerReiniging = DLookup("PrijsPerReiniging", "BedrijfPrijsContainerQ", "Bedrijven.FormaatContainer1= forms!bedrijfsgegevens!formaatcontainer1 and NaamOpdrachtgever= forms!bedrijfsgegevens!NaamOpdrachtgever")

Thank you very much for helping me.