What are the actual rowsource and control source for your combo boxes.

For instance, are they typical Relational Combo Boxes or are the values specified in a list in the control.

You might also want to check out Cascading Combo Boxes if each subsequent combo box's options are dependent upon the previous.

Start with your first combo box. Does the selection there effect the options in the next combo box? Does it effect the value you want to put in the text box? Then work from there setting or updating the field value as each combo is adjusted.

Hello Donald,

The ControlSource for the first combo = Trainer,
The rowsource = "Hoofdtrainer";"Clubtrainer";"Trainer";"Lid";"Losse Les"

The ControlSource for the second combo = JunSen
The rowsource = "Junior";"Senior"

ControlSource for the third combo = Week
The rowsource = "1x/week";"2x/week"

ControlSource for the fourth combo = Meer
The rowsource = "1 lid";"meer leden"

ControlSource for the fifth combo = Maand
The rowsource = "Maand";"Kwartaal"

The idea was to use it like a Karnaugh diagram

No, the selection in the every combobox should only have effect on the Textbox "Bedrag"

I'd put the pricing in a table instead of trying to build a giant nest of IIf statements. Create a table such as PriceT with these fields:

Trainer
JunSen
[Week]
Meer
Maand
Bedrag

Add one record for each valid combination and its price. You may also want a unique index across the first five fields so you can't accidentally enter the same combination twice.

Then put this procedure in your form's module. This assumes the combo boxes have the same names as their ControlSource fields:

Private Sub UpdateBedrag()
    Dim Criteria As String

    If IsNull(Trainer) Or IsNull(JunSen) Or IsNull([Week]) Or IsNull(Meer) Or IsNull(Maand) Then
        Bedrag = Null
        Exit Sub
    End If

    Criteria = "Trainer=""" & Trainer & """ AND JunSen=""" & JunSen & """ AND [Week]=""" & [Week] & """ AND Meer=""" & Meer & """ AND Maand=""" & Maand & """"
    Bedrag = Nz(DLookup("Bedrag", "PriceT", Criteria), 0)
End Sub

Then, in the After Update event for each of the five combo boxes, call that procedure:

Private Sub Trainer_AfterUpdate()
    UpdateBedrag
End Sub

Private Sub JunSen_AfterUpdate()
    UpdateBedrag
End Sub

Private Sub Week_AfterUpdate()
    UpdateBedrag
End Sub

Private Sub Meer_AfterUpdate()
    UpdateBedrag
End Sub

Private Sub Maand_AfterUpdate()
    UpdateBedrag
End Sub

Set each combo box's After Update event property to [Event Procedure].

This way, every time the user changes a selection, Access looks up the matching amount and puts it in Bedrag. If it returns 0, that means there isn't a matching combination in PriceT, which is also a handy way to spot a missing price record.

One small caution: if Bedrag is the actual amount charged to a customer, storing it with the customer record is usually correct. That preserves the historical price if you later change your prices in PriceT.