I went to the Thai Embassy in Denmark today and after reading this on their website I did not bring my plane ticket (stupid me):

• Your actual Passport or Travel Document.
  - Passport or Travel Document must not expire within 6 months and contain at least ONE completely empty visa page.
  
• 1 copy of Passport or Travel Document (Picture page)
  
• 1 application form
  - completed and signed by the applicant. Parents can sign for the minor.
  
• 1 photo
  - passport-type photo, size 4 X 5 cm., color, front-view, taken within 6 months
  - Photostat or Photocopy will not be accepted
  
• A copy of a valid and effective marriage certificate
  - A copy of Thai passport /or Thai I.D. of the spouse
  - A letter from the spouse verifying that the applicant and the spouse are still married and the purpose to visit Thailand.
  

I were told I would need to show a plane ticket as well, I have one but only a one way ticket as I believe I read here thats all thats needed. Now I am worried I would need a return ticket is that the case?

If not is there any documentation anywhere I can bring along in case they request it?

Thai Embassy notwithstanding, the plane ticket is required for the airline boarding process. A one-way ticket is ok, provided you have a valid visa for entry into Thailand. Just print out another copy of your electronic ticket for the Thai Embassy, if required.

I have entered on a one way ticket (twice) from th UK with no problem, as long as you have the correct visa you should be OK.

...A one-way ticket is ok, provided you have a valid visa for entry into Thailand...

For the visa application it really depends on the embassy/consulate where you apply. For example, the honorary consulate in Zurich, Switzerland, regularly asks for the copy of a return ticket. I tested this once by not enclosing this copy with my application for a non-O visa to visit family and when I heard nothing from them for a week I called and they told me that my application was not being processed because the copy of the return ticket was missing.

spacemand, I suggest that with the copy of your one-way ticket you also submit a covering letter addressed to the consul, explaining that after arriving in Thailand you plan to get an extension of stay to live with your wife in accordance with paragraph 2.18 of the Royal Thai Police Order No. 777/2521 and that you do not yet know when you will leave Thailand, for which reason you will travel to Thailand with a one-way ticket. This should stop the clerks from giving you the runaround.

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Maestro